Twin birthweight in Nigeria and the effect of sex-pair and parity.

In a retrospective study the mean birthweight, mean birthweight sum and their variation within parity and sex-pair are determined for 622 consecutive twin pairs among 17.726 deliveries conducted over an 18-month period at the University of Ilorin Teaching Hospital, Ilorin, Nigeria. Overall mean birthweight 2.5 +/- 0.5 kg, and birthweight sum 4.9 +/- 1.1 kg are obtained. First twin weighed 2.52 +/- 0.5 kg, slightly heavier than second twin, 2.48 +/- 0.5 kg. 55% of all twins weighed 2.5 kg or less. Male twins were heavier than females in unlike-sex pairs. Male co-twins weighed more than female co-twins, and unlike-sex twins weighed more than like-sex twins. Mean birthweight, and mean birthweight sum were lowest in para: 1, increasing gradually to a plateau in para: 4 and above. Several factors including preponderance of maleness and unlike-sex pairs in the twinning incidence in Nigeria, and the high proportion of our patients registered for antenatal care, may have positively influenced the overall mean birthweight.