Twinning in southern Israel; secular trends, ethnic variation and effects of maternal age and parity.

Twin births in southern Israel between 1970 and 1986 were examined in the Jewish and Bedouin populations. An increase in dizygotic twinning in the whole population, largely due to an increase of rate in the Bedouin population was found. The dizygotic twinning rate in the Bedouin population rose until it reached the level found in the Jewish population. No change with time was found in the monozygotic twinning rates in either population. This suggests that while dizygotic twinning rates are influenced by environmental factors, the monozygotic twinning rates are not. The effects of maternal age and parity on dizygotic and monozygotic twinning rates differed in the two ethnic groups examined. In the Jewish population the dizygotic twinning rate was related to maternal age and parity, while in the Bedouins only maternal age affects the rate. The monozygotic twinning rate has an inverted U shape with maternal age in the Jewish population and is linearly related to maternal age in Bedouin women. No effect of parity on the Jewish monozygotic twinning rate is found but this rate is directly affected by parity in Bedouin women. The effects of maternal age and parity together were examined in both populations. Both maternal age and parity affected the twinning rates; however, the effects are not additive and no interaction between maternal age and parity was found.