Seasonal variation in the time to pregnancy: a secondary analysis of three Danish databases

Three Danish databases were reanalysed to investigate seasonal variation in the time to pregnancy. Information was available on cohorts of women selected on the basis of union membership or residence in a given area: textile workers in Denmark (with 1,053 first and 1,771 second pregnancies), pharmacy assistants in Denmark (with 734 first and 725 second pregnancies) and pregnant women in the 36th week of pregnancy in two Danish cities (with 3,657 first and 3,526 second pregnancies). The influence of the season was of primary interest, because it is presumed to cause impaired ovarian function and hence a prolonged time to pregnancy. Furthermore, we studied whether the waiting time was prolonged in other situations with possibility of decreased ovarian function: in young and older women. In genera, seasonality in the time to pregnancy based on the time of conception was found with a higher chance of a prolonged waiting time before conceiving in February-April and a lower chance of a prolonged waiting time before conceiving in August-October. This association was not distorted by the age of the women or diabetes mellitus. A prolonged time to pregnancy was found in women of 30 years or older. Women of 20 years or younger did not have a prolonged waiting time, but most of them were well beyond the age of menarche and thus beyond the period of impaired ovarian function. On a population level, there was evidence for seasonality in the time to pregnancy, which is compatible with seasonal variation in pregnancy planning as well as with biological influences.