"Seasonality of Birth"
(Miura T. ed., SPB Academic Publishing, 1987)
Conclusion from each chapter
List of authors
Causes and effects of birth seasonality:
- 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'.
- 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.
- Effects of the later reinfection may be modified by the immunizing
effect of the first inapparent infection in fetal stages.
- These hypotheses may suggest the possible etiology of seasonal infertility,
schizophrenia, and some other etiologically unknown diseases.
- Some physiological functions and therir seasonal variations may also
be related to these hypotheses.
- 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
- Several problems in considering the nature of the birth month cohorts
- 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.
- 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.
- 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.
- The factors employed to separate the birth month cohorts, such as sex,
age, race, or other immunological or biological factors, were listed.
- 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.
- Three types and three variations of standard figures, their combinations,
and some other drawing rules used in this volume were described.
Recent changes in the seasonality of
- 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.
- 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.
- These facts may suggest that the seasonality of birth is not determined
by climate or temperature, and may be determined by some other mechanisms.
Secular changes in the seasonality of
T. Miura, J. Richter, M. Shimura and T. Ogata
- Seasonality of birth has chaned periodically in all the four countries
investigated, Japan, Germany, the United States and England.
- 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.
- In Japan, the fall peak was evident in both earlier halves of the 18th
and 19th centuries.
- 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.
- 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
Epidemic seasonal infertility.
T. Miura, M. Shimura, I. Nakamura and K.Nonaka
- 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
- These factors infect embryos and cause abortion. Only those who survived
the infection were born in the low-birth-rate 'May-July' season.
- 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.
- 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.
- Marriage-first birth intervals by month of the first birth, and by
the month of mother's birth, also supported this hypothesis.
- 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
- 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.
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
- The twinning rate since 1611 in Görlitz and since 1541 in London
has fluctuated secularly.
- 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.
- 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.
- The seasonal distribution of the twinning rate in Görlitz was
variable among decades investigated, while seasonal variation of the birth
ratio was almost stable.
- In summer and winter the twinning rate varied greatly through the decades,
although it was almost stable in spring and fall.
- 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.
Seasonality in twinning during 60 years
I. Nakamura, K. Nonaka, M. Shimura, Y. Amau, E.
Obata and T. Miura
- 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,
- Rather stable seasonality was observed in twinning during the nearly
60 years. The higher twinning rates were observed in the latter half of
- The seasonal variation was observed more evidently in like-sexed twinning
than in unlike-sexed twinning.
- The twinning rate was affected by the month of mother's birth.
- 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.
- 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.
- A possible explanation for these phenomena was given and a hypothesis
The Season of mother's birth as a factor
in changing the secondary sex ratio.
K. Nonaka, I. Nakamura, T. Miura and J. Richter
- The seasonal variation of the sex ratio at birth was not as stable
as had been generally believed.
- In baptism church records in Görlitz, east Germany, patterns of
seasonal variation of the sex ratio were different between the 17th and
- In hospital data in Tokyo, Japan, the season with a higher (or lower)
sex ratio changed during the years 1924-1980.
- 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
- 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
- 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.
Season and age at menarche.
T. Miura, I. Nakamura and M. Shimura
- In Japanese girls born since 1960, the menarche occurs most frequently
in August, January and April.
- 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.
- It was reconfirmed that girls tended to have menarche at the same season
as that of their own births.
- 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
- 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.
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
- Seasonal variations of menstrual frequency were different among girls
born in different seasons.
- The girls born in particular seasons have experienced menarche later
and showed lower menstrual frequencies after their maturation.
- 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.
- Serum LH and FSH concentrations in the longer menstrual-cycle-length
group showed a higher level near the day of ovulation, excluding the surge
- Serum estradiol concentration was lower in the longer menstrual-cycle-length
- Hypothetical explanations were offered to explain all these endocrine
phenomena as a whole.
- 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.
Season of birth and physical characteristics.
M. Shimura, K. Nonaka, I. Nakamura, Y. Suguri,
F. Fujikatsu, A. Ichiki, H. Kawana and T. Miura
- 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.
- 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.
- 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.
- Among athletic female students born between 1953 and 1957, those with
higher athletic ability were born less frequently in April-July.
- In these athletes higher stature was recognized also in those born
- Among the athletes born in May-July, when excellent athletes were born
less frequently, physical attributes were most different by athletic ability
- 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.
- Girls born in April-August had lower visual acuity and shorter legs.
- 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.
- 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.
Congenital malformations by month of
H. Kanai and I. Nakamura
- 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.
- 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.
- Monthly birth distributions of cases with mental retardation, cerebral
palsy and autism showed excesses in December-March.
- 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
Seasonalities of obstetric complications
and their relations to birth season of mother.
I. Nakamura and M. Uno
- We examined 3,271 case records of deliveries with 24 or more gestational
weeks at an obstetric clinic in a university hospital in Tokyo.
- Regarding the seasonality of deliveries with complications, the followings
- As for the seasonality of mother's own birth, the followings were observed:
- 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.
- 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
- 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.
Season of birth in perinatal abnormalities.
M. Shimura and T. Miura
- 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.
- 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'
Season of birth in mental disorders.
M. Shimura, T. Miura and T. Kimura
- Seasonal distribution of births of schizophrenics showed some deviation
from the control, and the pattern of the deviation shifted by periods of
- Seasonal births of manic-depressives also showed some deviation from
the control, and the pattern of the deviation shifted by periods of birth
- Seasonal births of neurotics did not show significant deviation from
- 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.
- Among the various explanations for the birth seasonality in schizophrenia,
the 'fetal infection and later reinfection' hypothesis seems to be the
Season of birth in parkinsonism.
T. Miura, M. Shimura and T. Kimura
- In an asylum for the aged in Tokyo, very high life-time morbidity (6.5%
or 6,500/100,000) of parkinsonism was found.
- Among them, males born in the earlier half of a year showed the highest
- 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).
- 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.
- 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
- 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.
Season of birth in some neurological
disorders -- multiple sclerosis, amyotrophic lateral sclerosis, senile dementia.
M. Shimura, T. Kimura and T. Miura
- 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).
- Male and female patients have different patterns in birth seasonality
in cases of MS and senile dementia, while the sexes have similar patterns
- 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.
- These phenomena suggest that these diseases could be related to etiological
agents which are active seasonally during fetal or perinatal stages.
Season of birth in convulsive disorders.
T. Kimura, M. Shimura and T. Miura
- 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
- 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.
- 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.
- 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.
- 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
- 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.
Longevity and geriatric diseases by
season of birth.
Y. Seko, M. Shimura, M. Sugamata, H. Kawana and
- 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
- Birth-seasonal variations of the rate of apoplexy and of systolic blood
pressure changed according to birth year.
- 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.
- 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.
- Birth-seasonal variation in the incidence of IHD was similar to that
of cerebral infraction rather than to that of cerebral hemorrhage.
- 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.
Cancers and season of birth -- analysis
of personal histories in breast cancer.
H. Nakao, M. Shimura and T. Miura
- 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.
- 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.
- 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
Relative frequencies of bone fractures
and dental caries by month of birth.
T. Miura, M. Shimura, T. Kimura, A. Ichiki and
- Relative frequencies of bone fractures and dental caries were related
to the month of birth.
- 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.
- Among cases of femoral fractures of the aged in two hospitals, the
number of those born in July-October was much more than expected.
- 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.
- Incidence of femur neck fractures in the aged in an asylum had increased
more than twice in recent decades.
- 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.
- 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.
Japanese encephalitis: different susceptibility
by season of birth.
T. Miura, T. Ogata and M. Sugamata
- 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.
- 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.
- 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.
- 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
- 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.
- 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
- 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.
Pediatric viral infections by month
T. Miura and M. Shimura
- Susceptibility to poliomyelitis was revealed to be different by season
- 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.
- 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.
Immunological effects of Japanese encephalitis
virus infection at various fetal stages in mice.
M. Sugamata and T. Miura
- 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.
- 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.
- 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.
- Fetal JE virus infection occurred in mice most frequently when the
virus was inoculated at the middle stage of pregnancy.
- The immunity of mice experimentally acquired in fetal stages could
be transferred to normal suckling mice by the serum in which no antibody