Cyclic variations of twin births at Isle-aux-Coudres: a case/control approach.

Twins and singletons were matched for several confounding factors. The monthly distribution of twin births and that of singleton births were compared with a uniform allocation of births over the year. Opposite seasonal variations emerged that were confirmed by a case/control comparison. Twins occurred more often in winter and early spring while singletons proved to be relatively few; singleton peaked in the fall season when the risk of a twin birth was low. These trends held across maternal age at birth and the time period of birth. Results suggest that the conception of a twin pair is highest in spring and early summer and lowest in winter. The role of sunlight in the twinning liability is discussed along with the role of sexual intercourse. That twin-prone mothers are usually more fecund in spring and early summer is a distinct possibility.