Twinning in New England in the 17th-19th centuries.

Vital records of Saybrook and Plymouth in New England from the 17th century were investigated. Among 8,562 maternities 81 twin maternities were found, the twinning rate being 0.95%. Twinning rate was low at the 1st and 2nd births as compared with the 3rd or later births, and was highest at the 7th and 8th births (1.6%). Twin maternity seemed to be a strong risk factor to terminate reproduction, particularly after 6 or more children had been delivered. The rate of mothers who had any other child ("fertile" mothers) at the 7th or later birth order was significantly lower for twin (13%) than for singleton maternities (63%). Twinning rate also varied by the size of offspring of a mother, and those mothers who had 5 or 6 children showed the highest twinning rate (1.3%). Those fertile mothers who had 7 or more children showed the lowest twinning rate (0.74%), although an exceptionally higher twinning rate was seen at their last births. Elongation of the last birth interval was observed for each group of every family size, and higher twinning rates were generally observed at their last births. Reduction in fecundity and rise in twinning rate seem to have occurred simultaneously at the last stage of the reproductive period of mothers, regardless of their family size.