More Babies Are Born In the Autumn: Is Sperm Quality to Blame?
Possibly, in part. A study of 6,455 semen samples found seasonal variation in sperm concentration, motility, and morphology, tied to scrotal temperature -- since sperm needs to stay a few degrees cooler than body temperature, and conceptions cluster in the cooler months that lead to the observed late-summer birth peak.
Ever wondered about seasonal birth patterns? In the US, more babies are born between July and September (meaning conception happened between October and December), with relatively fewer born in spring (conceived over summer).
Cuddling season, or sperm quality?
Do we simply cuddle up more in colder months? Probably a factor. But a well-designed study found it may also come down to seasonal variation in sperm concentration and quality. We know sperm needs to stay a few degrees cooler than body temperature -- when scrotal temperature climbs too high, sperm concentration, motility, and morphology all take a hit. So it's not especially surprising that a large study of 6,455 semen samples found genuine seasonal variation in sperm quantity and quality, which may partly explain the seasonal highs and lows in birth rates.
Reference
Levitas et al. (2013). American Journal of Obstetrics and Gynecology, 208(5), 406. Seasonal variations of human sperm cells among 6,455 semen samples: a plausible explanation of a seasonal birth pattern.
Common questions
When are the most babies born in the US, and when were they conceived?
More babies are born between July and September in the US, meaning conception mostly happened between October and December -- the cooler months.