The number of new-borns with an orofacial cleft exhibited regular seasonal variation; significantly more babies were born during March, April and May, while significantly fewer babies were born in October, November and December. Similar seasonal variation, without a significant difference from the control group, was also found in the whole group of new-borns with an orofacial cleft (CL, CLP, and CP). After subdividing the cleft patients according to gender and cleft type, there was only a significant increase in the total number of boys with CL born in August and of girls with CL born in March during a period of 37 years when compared to the control group. Conversely, a significant decrease was found in the number of boys with CP born in April and of girls with CL born in January.
Control group of new-borns
It is a general rule that more boys than girls are born regularly. Our data on the control group are in accordance with this rule (Fig.
2). In our country, only one anomaly has been reported; in November 1986, seven months after the Chernobyl nuclear accident and the attendant release of radiation, more than 450 new-born boys were missing. This implies that this month, for the first and only time during the last 50 years, more girls than boys were born in the Czech Republic [
6,
34].
During the regular seasonal decrease in the total birth number from April to December, a small peak appeared in the curves in September [
6] (see also Fig.
2). This small peak reflects conception in December. This phenomenon is called the “Christmas effect”, which has been observed in many countries, e.g., in Norway [
35], the USA [
36] and Croatia [
37].
Increase in the number of new-borns with CL
In comparison to the control group, we found only two significant peaks in the total monthly numbers calculated for 37 years: in girls with CL in March (critical period of cleft formation in July), and in boys with CL in August (critical period in December). No significant increase was detected in the other cleft types (CP, CLP) in comparison to the control group.
Edwards [
10] has tracked the seasonal variation in the proportion of children with various abnormalities standardized against the total birth number in Birmingham. Among 17 abnormalities, the maximal seasonal incidence was found in new-borns with CL in March.
A significantly above average seasonal incidence of CL, with or without CP, exists in the United States from November through March in the region characterized by hot summers and moderate winters. This might indicate that some factors prevalent in hot summer areas are involved in the malformation process [
12].
In Finland, Rintala et al. [
15] have observed seasonal variation in the number of new-borns with CL and CLP, with a peak in April, but no seasonal variation in the number of new-borns with CP.
Decrease in the number of new-born boys with CP and girls with CL
We found a significant decrease in the number of boys with an orofacial cleft in April. More detailed analysis revealed that this overall decrease is caused by a specific decrease in the number of boys with CP. However, this decrease did not result from a premature delivery of the missing children, since the decline was not preceded (compensated) by a peak in March. The absence of a portion of boys with CP can be explained from either a positive aspect - the CP did not arise, or from a negative aspect – the missing boys with CP were aborted. It is known that after prenatal exposure to a strong harmful factor, the number of malformed new-borns may decrease [
38] as a result of their prenatal abortion (for review see) [
39‐
41]. This effect concerns mainly the male fraction [
6,
34,
42‐
45].
The decrease in the mean number of CP boys in April was followed by an increase in May (Fig.
4). Such a down-up anomaly repeatedly appeared in 25 of the 37 years under investigation. Birth dates in April or May correspond to a critical period for cleft origin during August–September or September–October, respectively. Hypothetically, both the recurrent decrease in April and increase in May might be caused by an injurious factor, which is strong in the first case, with a recordable lethal effect, and which is no longer sufficiently strong in the latter case as to result in prenatal death, but strong enough to increase the number of malformations.
The decrease in the number of girls with CL in January and the subsequent increase in March can be interpreted in a similar way (Fig.
5).
Future studies should focus on elucidating the above-mentioned down-up anomalies.
Putative harmful factors
In the present study, the timing of the effect of putative harmful factors was considered with regard to the critical period of cleft formation (Fig.
1) and the fact that both a significant decrease or increase in the number of clefts can signal prenatal damage of the embryos – death or malformation, respectively (see above). Seasonal variation in the number of inborn defects may indicate exposure of the mother to a harmful environmental agent (e.g., climatic changes, infections, dietary habits) whose presence varies through the year [
46]. Influenza and other respiratory viral infections (common cold) exhibit seasonality from early autumn to spring [
47,
48]. Infectious diseases accompanied by fever and/or drug intake in a pregnant woman have been reported as important risk factors for the development of an orofacial cleft in the embryo, if such factors are present during the first trimester of gravidity [
11,
20,
49‐
53]. Regarding the critical period of cleft formation, seasonal respiratory viral infections might contribute as risk factors to the increase in the number of new-born boys with CP in May (critical period in September – October), (Fig.
4). The significant increase in the number of new-born boys with CL in August corresponding to a critical period in December (Fig.
4) might be related to autumn respiratory infections and/or psychological stress experienced by pregnant mothers at Christmas time.
The minimum and maximum values (the abovementioned down-up anomaly) in the numbers of girls with CL born in January and March, respectively, corresponded to conception during April to June (Fig.
5) and the predicted critical period for CL formation from May to July. The number of boys with CP reached minimal and maximal values in April and May, respectively, meaning that conception took place during July to August (Fig.
4) and that the critical period for CP formation occurred from August to October. Taken together, these results indicate that the abovementioned boys and girls passed the critical period of cleft formation during May to October. From May to October, there are factors that could act either individually or in combination to impair developing embryos. This warm season is mainly characterized by high temperatures, sunshine and increased levels of UV radiation, agricultural pollutants, and ozone concentrations. A correlation between the incidence of CL in girls and the intensity of UV light has been reported, and conception in winter has been recommended as a preventive measure against CL formation [
20]. There is already extensive evidence of a wide spectrum of harmful health effects resulting from air pollution, including ozone levels [
54]. Outdoor exposure to air ozone during the first two months of pregnancy may increase the risk of orofacial clefts [
55]. Ozone is a secondary pollutant generated by photochemical reaction between volatile organic compounds (VOC) from biogenic and anthropogenic sources, NOx (produced mainly by traffic) and solar radiation; this reaction becomes more intense with increasing outdoor temperature [
54,
55].
In the Czech Republic, the ground-level ozone exhibits periodic seasonal variation with the highest values observed from April to September and minimum values from November to February [
56] (see Additional file
1). Similar seasonal variation is exhibited by the values of UV radiation [
57] and outdoor temperature [
58], (see Additional files
2 and
3). Collectively, these data suggest that the outdoor temperature, intensity of UV radiation, and levels of ground ozone, all of which reach maximum values during the warm season, might act as harmful environmental factors implicated in seasonal changes in the birth rate of babies with an orofacial cleft. In addition to the abovementioned environmental factors of the warm season, psychological stress associated with the summer holiday might initiate the stress response, including the elevation of corticoids [
59] in the maternal organism. Corticoids are known to induce orofacial clefts experimentally [
60,
61].
Study strengths and limitations
The strength of the study consists of analyses of a large sample of cleft patients (5619) collected over 37 years. This sample comprises all children born with an orofacial cleft in the Bohemia region of the Czech Republic during 1964–2000. The register is complete thanks to the centralized multidisciplinary treatment of the patients in the Cleft Centre at the Plastic Surgery Clinic, Prague, Czech Republic.
The register at the clinic naturally includes only live-born children, which is why we had to use national data on live births only.
The presently used sample of the birth rate in the Bohemia region contains all live births (3,080,891) during the period 1964–2000, including prematurely born children and children with major inborn anomalies - data on the premature births or major birth defects were not available. Therefore, we used the sample of cleft patients without further selection criteria.
Since the mean incidence of all major birth defects in the children born in our country is 340.90/10,000 [
62], we assume their inclusion in our control group should have no potential impact on the seasonality of the birth-rate data. Nevertheless, for the evaluation of the birth rate in the cleft group, the control group was formed by subtracting the cleft patients from the total number of live new-borns to obtain the control sample only including the new-borns without an orofacial cleft.
With regard to the prematurely born children, some misclassification of conceptions might occur by the inclusion of premature birth. However, such a misclassification could not significantly influence the results; the results are based on large sets of cumulative data for 37 years. This is documented by the regular course of the curves in the control group (Fig.
2). Furthermore, the putative misclassification would similarly concern all control and cleft patient groups.
In the groups of cleft patients, a misclassification could not explain the significant decrease in the number of boys with an orofacial cleft in April, since this decrease was not compensated by a peak in March, reflecting premature delivery of the missing children (Fig.
4). Vice versa, the significant peak in the numbers of boys and girls with CL in August and March, respectively, was followed by no decrease the following month (Figs.
4 and
5). This finding implies that these peaks do not reflect prematurely born children that are missing among new-borns a month later.