International Journal of Gynecology and Obstetrics 80 (2003) 255–261 0020-7292/03/$ - see front matter  2002 International Federation of Gynecology and Obstetrics. Published by Elsevier Science Ireland Ltd. All rights reserved. PII:S0020-7292 Ž 02 . 00381-8 Article Effect of prolongation of pregnancy on perinatal mortality H.E. Onah* Department of Obstetrics and Gynaecology, University of Nigeria Teaching Hospital, Enugu, Nigeria Received 18 March 2002; received in revised form 25 October 2002; accepted 30 October 2002 Abstract Objectives: (1) To determine the distribution of singleton and twin births according to gestational age in a Nigerian obstetric population; and (2) to compare their perinatal outcomes according to gestational age. Methods: A 10-year retrospective comparative study of twin and singleton births at a tertiary care center in Enugu, Nigeria. The variables analyzed were: the proportion of deliveries occurring at each gestational age, the gestational age-specific rates for stillbirths, cesarean section, babies with 1-min Apgar scores less than 4 and those whose birthweights were below the 10th percentile for gestational age. The trends in these rates were determined by finding the best fitting polynomial regression curve for each variable. Tests of statistical significance for trends in proportions were carried out by means of the x -test at the 95% confidence level. Results: Of the 496 twin births, 3.6% compared with 17.3% of the 496 2 singleton births went beyond 40 weeks’ gestation while 1.2% of the twin and 4.4% of the singleton deliveries occurred at 42 weeks’ gestation or beyond. For twins as well as singletons, there was a consistent and significant decline in the stillbirth rate and the proportion of babies with 1-min Apgar scores less than 4 up to 42 weeks (Ps 0.0000). Among the twins, the proportion of babies with birthweights below the 10th percentile (i.e. those with impaired growth) significantly rose from 28 weeks and above (Ps0.0000) while among the singletons, a declining trend with gestational age was observed (Ps0.0003). However, among the twins with impaired growth, the stillbirth rate neither differed between the first and second twins at each gestational age nor did it increase with gestational age in both the first and second twins. While the cesarean section rate for singletons remained almost stable at approximately 13%, there was a significant rise in the cesarean section rate with gestational age among the twin births. Conclusions: There were 1.2% of twin deliveries compared with 4.4% of singleton deliveries which occurred at 42 weeks’ gestation or beyond. In the Nigerian population studied, the perinatal outcomes in twins did not differ from those of singletons up to 42 weeks’ gestation suggesting that the 42-week cut-off for prolonged pregnancy applies equally well to twins as to singletons.  2002 International Federation of Gynecology and Obstetrics. Published by Elsevier Science Ireland Ltd. All rights reserved. Keywords: Prolonged pregnancy; Singleton; Twin; Perinatal outcome *Fax: q234-042-251978. E-mail address: hyacinon@infoweb.abs.net (H.E. Onah).