Seasonal birth pattern in Sweden in relation to birth order and maternal age.

The influence of both maternal age and parity on seasonal birth patterns in Sweden was studied, using the Swedish Medical Birth Register for 1973. The register contained reports of 109 342 pregnancies. A test for seasonality, using a squared sinus function and an analysis of variance, was performed for each age and parity group. Significant differences were shown for parity irrespective of age and age groups within parity groups. There was one exception: no influence of parity could be demonstrated in women more than 35 years of age. For the lowest and the highest age groups (all parities), there was no significant seasonality in birth rate tested by the squared sinus method. This was also true for parity 3 (age group 20--24) and parity 1 (age group 30--34). The other age groups and parity classes show a significant seasonality with a maximum of births in April and May.