Seasonal factors may be responsible, says lead study author Amelia Wesselink, postdoctoral researcher in epidemiology at Boston University.
Lower vitamin D levels in the winter and spring reduce sperm quality and egg supply, she explains. Meanwhile, air pollution spikes in the winter thanks to burning fireplaces and cars that are left idling to stay warm. This can increase levels of oxidative stress and inflammation, further hurting fertility.
The bottom line:
If you or your partner is trying to get pregnant, ramp up your efforts in late summer or early fall. November and December were peak conception periods, per the study.