"Seasonality of Birth"

(Miura T. ed., SPB Academic Publishing, 1987)

Conclusion from each chapter

List of authors


Chapter 1

Causes and effects of birth seasonality: general considerations.

T. Miura

  1. A hypotheses for the cause and effects of birth seasonality is proposed. The principal hypotheses is the seasonal 'inapparent fetal infection' followed by the seasonal 'later reinfection'.
  2. The initial inapparent infection may cause minor changes in infected fetal tissues, which will leave some dysfunction, or will develop into more severe sequalae at a later time.
  3. Effects of the later reinfection may be modified by the immunizing effect of the first inapparent infection in fetal stages.
  4. These hypotheses may suggest the possible etiology of seasonal infertility, schizophrenia, and some other etiologically unknown diseases.
  5. Some physiological functions and therir seasonal variations may also be related to these hypotheses.
  6. Steps to confirm the hypotheses are briefly considered and possible effects of these hypotheses are suggested in a few cases.


Principles in methods of epidemiological studies on birth seasonality.

K. Nonaka and T. Miura

  1. Several problems in considering the nature of the birth month cohorts were summarized.
  2. When pooling the birth month cohorts over a long period of years or a wide range of birth places, we should ensure homogeneity of the data, because we suppose an underlying factor whose prevalence may differ form time to time and from place to place.
  3. When we examined qualitative data, the positive rate was calculated for each birth month cohort, and tested statistically by the É'2 test. If the information on the population could not be obtained, monthly distribution of the number of general births was used as a control. When we examined quantitative data, the mean value on each birth month cohort was used, and tested by the t-test or analysis of variance.
  4. The monthly or seasonal patterns of seasonally changing phenomena were compared among several birth month (or season) cohorts. This mode of analysis is useful for suggesting the environmental factors which may have effects only in a particular season.
  5. The factors employed to separate the birth month cohorts, such as sex, age, race, or other immunological or biological factors, were listed.
  6. The tests for seasonality used in this volume were explained. Those methods which have been used by other reserchers, such as Edwards' test, were considered inappropriate for the purpose of our study because of their assumption of a simple sinusoidal variation.
  7. Three types and three variations of standard figures, their combinations, and some other drawing rules used in this volume were described.

Chapter 3

Recent changes in the seasonality of birth.

T. Miura

  1. The birth seasonality with a spring peak, which has been considered to be a basic rhythm of human reproduction, has changed in the late 20th century quite rapidly, not only in Japan, but in other countries as well. In Tokyo the change happened rather abruptly in 1964.
  2. In Hawaii and West Malaysia, the seasonal birth patterns are not the same among different ethnic groups. In Thailand, conceptions occurred most frequently in the hottest month there, April.
  3. These facts may suggest that the seasonality of birth is not determined by climate or temperature, and may be determined by some other mechanisms.

Chapter 4

Secular changes in the seasonality of birth.

T. Miura, J. Richter, M. Shimura and T. Ogata

  1. Seasonality of birth has chaned periodically in all the four countries investigated, Japan, Germany, the United States and England.
  2. In general, the peak of births was in the spring, or the January-March season. However, in some decades the birth peak in the fall (September-November) exceeded the spring peak.
  3. In Japan, the fall peak was evident in both earlier halves of the 18th and 19th centuries.
  4. The fall peak of birth was most prominent during the 16th century in England and during the 17th century in East Germany and during the late 18th century in the United States and East Germany.
  5. These alternative secular changes in birth seasonality observed in all four countries suggest that the so-called 'basic animal rhythm' or the seasonal variation of human reproductive ability could be a result of environmental conditions, which could change drastically and alternate with time.

Chapter 5

Epidemic seasonal infertility.

T. Miura, M. Shimura, I. Nakamura and K.Nonaka

  1. To esplain global variations in the sesonality of birth we proposed a hypothesis that the low-birth rate season is caused by factors of 'epidemic seasonal infertility'.
  2. These factors infect embryos and cause abortion. Only those who survived the infection were born in the low-birth-rate 'May-July' season.
  3. These survived female babies will be immune to the infertile factors when they are adults and become pregnant. Therefore, they will have babies evernly throughout a year, while all the other mothers will show evident seasonality of pregnancy due to the infection by the infertile factors.
  4. The birth records of two maternity hospitals and a group of questionnaires supported this hypothesis for the 1930's, when birth seasonality was most prominent in Japan.
  5. Marriage-first birth intervals by month of the first birth, and by the month of mother's birth, also supported this hypothesis.
  6. The seasonal distribution of the second (or the third) births was also influenced by the season of the first (or the second) births, and supports this hypothesis.
  7. Some populations in the central African countries, and the Hutterites, are supposed to be examples of the populations heavily-, and not-, contaminated by the infertile factors, respectively.

Chapter 6

Secular changes in the seasonal twinning rate since 1611 in Görlitz, Germany, and since 1541 in London, England.

K. Nonaka, I. Nakamura, J. Richter and T. Miura

  1. The twinning rate since 1611 in Görlitz and since 1541 in London has fluctuated secularly.
  2. The mid 17th century and the latter 18th century in Görlitz, and the mid 16th, the early 17th, the late 17th and the early 18th centuries in London, were periods with a higher twinning rate during the period investigated.
  3. When the twinning rate in London was separated into like-sexed and unlike-sexed, the secular fluctuation of the unlike-sexed twinning rate was different from that of the total twinning rate.
  4. The seasonal distribution of the twinning rate in Görlitz was variable among decades investigated, while seasonal variation of the birth ratio was almost stable.
  5. In summer and winter the twinning rate varied greatly through the decades, although it was almost stable in spring and fall.
  6. Seasonally epidemic biological factors were suggested to explain the secular changes and seasonal variations of the twinning rate.

Note: we carried out a further investigation on the baptism records in London after having finished this manuscript. Suggestions from the investigation were basically the same as presented here.

Chapter 7

Seasonality in twinning during 60 years in Japan.

I. Nakamura, K. Nonaka, M. Shimura, Y. Amau, E. Obata and T. Miura

  1. Case records of two hospitals and registration records of an association of twins' mothers were analyzed. Years of twin births were 1924-1980 and there were 656, 109 and 1,185 twin maternities in the three sets of data, respectively.
  2. Rather stable seasonality was observed in twinning during the nearly 60 years. The higher twinning rates were observed in the latter half of the year.
  3. The seasonal variation was observed more evidently in like-sexed twinning than in unlike-sexed twinning.
  4. The twinning rate was affected by the month of mother's birth.
  5. The twinning rate was low among the mothers born in May-July in maternities up to 1960, and these mothers did not show seasonal variation of twin births, compared with the other mothers.
  6. The unlike-sexed twin rate (U-S twin rate) of the mothers born in May-July was also lower than that of the other mothers, and was consistently low even at the higher maternal ages.
  7. A possible explanation for these phenomena was given and a hypothesis was proposed.

Chapter 8

The Season of mother's birth as a factor in changing the secondary sex ratio.

K. Nonaka, I. Nakamura, T. Miura and J. Richter

  1. The seasonal variation of the sex ratio at birth was not as stable as had been generally believed.
  2. In baptism church records in Görlitz, east Germany, patterns of seasonal variation of the sex ratio were different between the 17th and 19th centuries.
  3. In hospital data in Tokyo, Japan, the season with a higher (or lower) sex ratio changed during the years 1924-1980.
  4. The sex ratio of the first-born babies under 25 years of maternal age varied by the month of mother's birth coincidentally both in the hospital and the questionnaire groups. In 1924-1928 and 1933-1937 the mothers born in March-May showed a higher sex ratio, while in 1929-1932 those born in December-February did so. Those shifts were observed similarly in the two groups.
  5. Some seasonally epidemic sex ratio-changing factors were hypothesized to explain the observed phenomena. The hypothesized factors were assumed to have a potency to decrease the sex ratio, as deduced from observation of the monthly variation in the sex ratio of mothers born in each of the twelve months.
  6. A study of students in Görlitz also showed the possibility of the dependency of the sex ratio of children on the month of mother's birth.

Chapter 9

Season and age at menarche.

T. Miura, I. Nakamura and M. Shimura

  1. In Japanese girls born since 1960, the menarche occurs most frequently in August, January and April.
  2. The month with the most frequent occurrence of menarche was different by the age of girls at menarche. Girls who had menarche at later ages had it more frequently in January-April than in August.
  3. It was reconfirmed that girls tended to have menarche at the same season as that of their own births.
  4. Age at menarche in recent Japanese girls is about 12.5-12.8 years. The acceleration of menarcheal age seems to have ceased among girls born after 1965.
  5. Menarcheal age was also different by month of birth. However, the direction of deviation by each birth season from the average age changed alternately through several decades.

Chapter 10

The length of menstrual cycle and its relations to menarcheal age, sex ratio of offspring and serum gonadotropin.

I. Nakamura, K. Nonaka and T. Miura

  1. Seasonal variations of menstrual frequency were different among girls born in different seasons.
  2. The girls born in particular seasons have experienced menarche later and showed lower menstrual frequencies after their maturation.
  3. Sex ratio of offspring of mothers who showed a longer menstrual-cycle-length was lower than those who showed a shorter-cycle-length. The sex ratio of offspring (high-low) and the length of menstural-cycles (short-long) showed a similar seasonal variation when distributed by the mother's birth month.
  4. Serum LH and FSH concentrations in the longer menstrual-cycle-length group showed a higher level near the day of ovulation, excluding the surge time.
  5. Serum estradiol concentration was lower in the longer menstrual-cycle-length group.
  6. Hypothetical explanations were offered to explain all these endocrine phenomena as a whole.
  7. Seasonal, environmental and exogenous factors which act in fetal or perinatal stages and characterize the ovarian functions in later life, were also suggested ato explain the relations of these phenomena to the month of birth.

Chapter 11

Season of birth and physical characteristics.

M. Shimura, K. Nonaka, I. Nakamura, Y. Suguri, F. Fujikatsu, A. Ichiki, H. Kawana and T. Miura

  1. Among 8,376 female college students born in 1941-1964 in Okayama, Japan, different secular trends were observed in the four measurements of body structure of height, weight, sitting height and chest circumference.
  2. Birth monthly variation of the four measurements was found to be greater among those born by 1950, when the trend and cycle factor was eliminated by the application of a time-series analytical method (CENSUS). The stature was higher among the students born in summer.
  3. This variation by the month of birth was temporarily reduced, and then increased again among those born in recent years, with different peak or trough months of birth.
  4. Among athletic female students born between 1953 and 1957, those with higher athletic ability were born less frequently in April-July.
  5. In these athletes higher stature was recognized also in those born in summer.
  6. Among the athletes born in May-July, when excellent athletes were born less frequently, physical attributes were most different by athletic ability level.
  7. Among 2,416 high school girls, the rate of girls having better visual acuity was higher among the girls who were taller, had longer legs and had lower values of the Rohrer index.
  8. Girls born in April-August had lower visual acuity and shorter legs.
  9. Among 581 pre-school children, the rate of those whose first-erupted permanent teeth were the central incisors (I-type) exceeded those whose first erupted were the molar (M-type). The rate of the I-type was different by birth season.
  10. These facts may suggest that some physical traits of man, which have been believed to be genetically predisposed, could be modified by seasonally changing environmental factors at a very early stage of life.

Chapter 12

Congenital malformations by month of birth.

H. Kanai and I. Nakamura

  1. Among 291 cases of Down's syndrome, a disproportionately large number was born in July-September. Twenty of 51 mothers who were delivered of a baby with Down's syndrome were born in January-February.
  2. Clustering in births of the cases with sex-chromosome abnormalities was observed during January-March and July-August for Klinefelter's syndrome, and during June-August for Turner's syndrome.
  3. Monthly birth distributions of cases with mental retardation, cerebral palsy and autism showed excesses in December-March.
  4. The number of births of children with abnormal appearance without chromosomal abnormality (AA) greatly increased in 1975, and three peaks were observed in the monthly distribution in that year. This annual pattern of birth coincides with that of the incidence of typhoid fever, and the seasonal birth patterns also coincide well with those of typhoid fever eight months earlier.

Chapter 13

Seasonalities of obstetric complications and their relations to birth season of mother.

I. Nakamura and M. Uno

  1. We examined 3,271 case records of deliveries with 24 or more gestational weeks at an obstetric clinic in a university hospital in Tokyo.
  2. Regarding the seasonality of deliveries with complications, the followings were observed:
  3. The pre-term and post-term deliveries occurred most frequently in summer, and in winter-spring deliveries, respectively, but not significantly.
  4. Significantly lower birth weights were observed in babies born in February-April.
  5. Monthly incidences of weak pains, premature rupture of the menbranes, and meconium stained amniotic fluid (MSAF) showed significant seasonal variations, with peaks generally in spring and fall and low troughs in summer, while those of prolonged labor showed no significant differences.
  6. The frequency of postpartum hemorrhage was also lower in deliveries in summer-fall (August-November).
  7. As for the seasonality of mother's own birth, the followings were observed:
  8. Mothers born in March-July complicated the post-term delivery more frequently, and they showed especially high incidences when they had a child in April-June.
  9. The babies delivered from the mothers born in March-June were heavier than those delivered from the other mothers. The seasonal difference of birth weight of babies was less apparent in the mothers who were born in March-June.
  10. Mothers born in summer showed a lower frequency of weak pains and related complications such as prolonged labor, premature rupture of the membranes and MSAF, and also showed a lower frequency in postpartum hemorrhage.

Chapter 14

Season of birth in perinatal abnormalities.

M. Shimura and T. Miura

  1. Perinatal deaths were more frequent among the summer born babies in the 1930's, in Tokyo. Perinatal death rate was influenced not only by the babies' birth season, but also by the season of their mothers' birth, suggesting the intervention of some immunological processes.
  2. Low birth weight was more frequent in the winter births only among the first born babies. No difference was found by the season of their mothers' births.

Chapter 15

Season of birth in mental disorders.

M. Shimura, T. Miura and T. Kimura

  1. Seasonal distribution of births of schizophrenics showed some deviation from the control, and the pattern of the deviation shifted by periods of birth years.
  2. Seasonal births of manic-depressives also showed some deviation from the control, and the pattern of the deviation shifted by periods of birth years.
  3. Seasonal births of neurotics did not show significant deviation from the control.
  4. The periods with larger deviation of neuro-psychiatric patients' birth distribution roughly corresponded to the periods with greater changes of seasonal temperature, although the influence of seasonal temperature was considered to be indirect.
  5. Among the various explanations for the birth seasonality in schizophrenia, the 'fetal infection and later reinfection' hypothesis seems to be the most appropriate.

Chapter 16

Season of birth in parkinsonism.

T. Miura, M. Shimura and T. Kimura

  1. In an asylum for the aged in Tokyo, very high life-time morbidity (6.5% or 6,500/100,000) of parkinsonism was found.
  2. Among them, males born in the earlier half of a year showed the highest morbidity (13%).
  3. In 2,571 parkinsonism and Parkinson's disease cases from a nation-wide surveillance, the sex ratio pattern by month of birth also showed a similar type among those who were born in the same years as the asylum cases, and also had higher sex ratio (1.32).
  4. But in younger cases born between 1901 and 1925, the sex ratio was lower than 1.0, and the sex ratio pattern by birth-month was also different from the elder cases.
  5. Among those born after 1926, however, the sex ratio pattern by birth-month was similar again to the eldest group, and the sex ratio was also high (1.35).
  6. Parkinsonism including Parkinson's disease was hypothesized to be related to an inapparent epidemic of some viral agent which may have been related to Economo's lethargic encephalitis.

Chapter 17

Season of birth in some neurological disorders -- multiple sclerosis, amyotrophic lateral sclerosis, senile dementia.

M. Shimura, T. Kimura and T. Miura

  1. From preliminary studies on a small number of cases, deviations of birth seasonality were detected in MS (213 cases), ALS (364 cases) and senile dementia (616 cases).
  2. Male and female patients have different patterns in birth seasonality in cases of MS and senile dementia, while the sexes have similar patterns in ALS.
  3. Age at the first medical examination was also different according to their season of birth. In MS and ALS patients, those born in the summer suffered from the diseases at younger ages by a few years than the others.
  4. These phenomena suggest that these diseases could be related to etiological agents which are active seasonally during fetal or perinatal stages.

Chapter 18

Season of birth in convulsive disorders.

T. Kimura, M. Shimura and T. Miura

  1. Birth-seasonality in febrile convulsion (FC) patients showed a spring-excess in patients aged less than 2 years and , a fall-excess in those aged over 2years.
  2. Onset-seasonality in FC patients showed spring and fall excesses among those aged less than 12 months, but only the spring-excess was observed among those aged 12 months or more.
  3. FC patients were apt to suffer from attacks in the season about one month after the month of their birth, namely in the season they had not experienced during their fetal stages or in the season they had experienced right after their birth.
  4. Birth-seasonality of epileptic (EPI) patients showed marked deviations in both female patients aged 10 to 30 years, and male patients aged 17 to 30 years born in 1951-1970, who supposedly consist mainly of idiopathic cases. However, the EPI patients aged less than 10 years or over 30 years, who supposedly include more symptomatic cases, showed little deviations.
  5. Birth-seasonality of FC-EPI patients showed a prominent spring-excess, which is similar to the one in EPI patients who are supposedly composed mainly of idiopathic EPI, and to the one in FC patients aged less than two years.
  6. It is possible that seasonal factors had influenced on FC and EPI patients in pre- or perinatal period, and that different factors may be responsible for cases different in the age at onset. On EPI patients, the factors might be also different by birth years.

Chapter 19

Longevity and geriatric diseases by season of birth.

Y. Seko, M. Shimura, M. Sugamata, H. Kawana and T. Miura

  1. Longevity was different according to the month of birth. In the male aged population, the survival rate was lower in those born in May-July than those born in the other months. This difference was not observed in the females.
  2. Birth-seasonal variations of the rate of apoplexy and of systolic blood pressure changed according to birth year.
  3. The change according to birth year in birth-seasonal variation also was observed in the rate of calcification of the aortic arch. This rate was significantly lower in those born around May-July than those born in the other months both in males and females.
  4. A similarity was observed in the birth-seasonal variation between cerebral infraction and cerebral hemorrhage in males born until 1900, but a dissimilarity between them was observed in both males and females born in 1901-1930.
  5. Birth-seasonal variation in the incidence of IHD was similar to that of cerebral infraction rather than to that of cerebral hemorrhage.
  6. In the present investigations, the possibility was suggested that the birth month influences the risk of cerebrovascular- and cardiovascular-diseases as well and consequently the longevity of the aged.

Chapter 20

Cancers and season of birth -- analysis of personal histories in breast cancer.

H. Nakao, M. Shimura and T. Miura

  1. The birth seasonality of the cancer patients was found to differ according to the site of the lesion. This finding suggests that various seasonal factors prevailing in the environment and acting during the fetal or neonatal stages may influence the morbidity of cancer at various sites.
  2. Rectum, colon and breast cancer patients had almost identical distributions in the seasonality of birth, especially in the younger patients. This result suggests that the cancers in these three sites might have, at least paritally, some epidemiologically similar etiological factors.
  3. The birth seasonalities of the post-menopausal and the pre-menopausal breast cancer patients were different. There might be at least two different etiological processes in pre-and post-menopausal breast cancer, both of which were derived from the infection by some agents at pre- or perinatal periods.

Chapter 21

Relative frequencies of bone fractures and dental caries by month of birth.

T. Miura, M. Shimura, T. Kimura, A. Ichiki and I. Nakamura

  1. Relative frequencies of bone fractures and dental caries were related to the month of birth.
  2. Among female students of a college for physical education, and male students, for both of whom the rate of bone fractures was high (20-22%), those born in July-October had higher frequencies of bone fractures (24-29%). However, among female students of general schools, whose rate of bone fractures was low (12%), the same phenomenon was not observed.
  3. Among cases of femoral fractures of the aged in two hospitals, the number of those born in July-October was much more than expected.
  4. Fractures related to the month of birth seems to be dependent on the fragility of bones. But fractures not related to the month of birth seem to happen by accidents or chance.
  5. Incidence of femur neck fractures in the aged in an asylum had increased more than twice in recent decades.
  6. The number of dental caries was found to be greater in those born in July-October, or in the latter half of a year, among adults patients and also pre-school children.
  7. It is suspected that month of birth is related to the consisitency of bones and teeth in young students as well as in aged persons.

Chapter 22

Japanese encephalitis: different susceptibility by season of birth.

T. Miura, T. Ogata and M. Sugamata

  1. Birth dates of JE patients were collected from 2,831 case records in five hospitals in the Tokyo area, and from 4,092 national surveillance cards detailing personal histories.
  2. The birth-month distribution of JE cases in 1965-1970 reviewed in the surveillance cards was distinctly different from that of the control general population when the patients were under the age of 15.
  3. The birth-month distribution of JE cases in 1948-1964 investigated in hospital records was also different among the cases who were born in the years when JE was epidemic.
  4. In both cases, in general, those born in the summer or around the JE epidemic season, tended to be more frequently found among the JE cases than expected from the birth-month distribution of the control general population.
  5. Accorging to the surveillance cards, the rate of the JE vaccinated was found to be about 20%. The rate of the vaccinated was also different according to the season of birth. Those born in the summer had the higher rate of the vaccinated when they were born in JE epidemic years.
  6. HI antibody positive rate was lower in persons born in July-October or JE epidemic season, especially in those born in south Japan, or JE epidemic area.
  7. In general, the susceptibility to JE virus infection and the effect of the JE vaccine was different according to the month of birth, especially for those who were born in JE epidemic years.

Chapter 23

Pediatric viral infections by month of birth.

T. Miura and M. Shimura

  1. Susceptibility to poliomyelitis was revealed to be different by season of birth.
  2. Measles history rate among children aged 5-6 years was much different by birth season in the birth cohort born in the measles epidemic year.
  3. The preventive efficacy of measles vaccination was also lower in the same birth-month cohorts born in the epidemic year with higher susceptibility or a higher rate of measles history.

Chapter 24

Immunological effects of Japanese encephalitis virus infection at various fetal stages in mice.

M. Sugamata and T. Miura

  1. In order to find experimental evidence to explain the different susceptibility by month of birth in JE cases, pregnant mice were infected with JE virus at various stages of pregnancy. The offspring from mothers infected at the middle stage of pregnancy were the most resisitant to the later challenge infection, even 91-180 days after birth.
  2. By cross fostering experiments, the mice from infected mothers (U group) were less resistant than the mice suckled by the infected mothers (M group). However, in the U group, those from mothers infected at the middle stage of pregnancy were the most resisitant.
  3. Serum antibody (HI and NT) titers were negative in the U group mice at age 16-20 days when they were completely (U-II) or partially (U-III) resistant. However, antibodies were detected in the same groups of mice 91-180 days after birth.
  4. Fetal JE virus infection occurred in mice most frequently when the virus was inoculated at the middle stage of pregnancy.
  5. The immunity of mice experimentally acquired in fetal stages could be transferred to normal suckling mice by the serum in which no antibody was detected.