Twinning rate in a sample from a Brazilian hospital with a high standard of reproductive care

Abstract

CONTEXT: Epidemiological studies on twin births have been motivated mostly by the positive correlation between twinning rate and human fertility, prematurity, low birth weight, increased risk of infant death and long term risk for morbidity. OBJECTIVE: This paper intends to estimate the incidence of multiple births in a private hospital in Brazil with a high standard of reproductive care, and to evaluate the effects of maternalage,gestation order and assisted fertilization on twinning rate. DESIGN: Retrospective analysis. SETTING: First-class tertiary private hospital, São Paulo, Brazil. PARTICIPANTS: The multiple birth rate was investigated among 7,997 deliveries from 1995 to 1998, including 7,786 singletons, 193 twins, 17 triplets and one quadruplet. RESULTS: Theratesper1,000dizygoticandmonozygotic pairs and for triple tswere estimatedas 19.51,4.50 and 2.13,respectively.The dizygotic and tripletrates were the high est observed in Brazil up to the present day.Thet winnin grate among primi gravidae older than 30 years was very high (45.02 per 1,000) and was dueto a disproportionately high frequency of dizygotic pairs.Thet riple trate was also very high among the mothers of this age group (5.71 per 1,000). These facts are strong indicators that these women were the one smost frequently submitted to assisted reproductive techniques. The mean maternal age of the studied population was about six years high erthanthat estimated for mothers in the general population of south eastern Brazil. Primigravidae aged under 30 years as well as multigravidae showed similar twinning rates, which were almost 20 per 1,000. Among the deliveries of multigravidae older than 30 years, an unusually high frequency of monozygotic twins was observed (7.04per1,000), probably as aconsequence of the residual effect of long-term use of oral contraceptives. CONCLUSIONS: The dizygotic twinning rate increased from 13.51 to 28.98 per 1,000over the four years studied,with the twinning rate for primigravidae over 30 years old in 1998 being twice that observed in 1995.The mean maternal age was also high during this period, but the extremely high in crease in twinning rate observed cannot be attributed solely to this variable. Assisted fertilization seems tobe the most probable cause of this un usually hight winningrate.