EPIDEMIOLOGIC AND BIRTH-WEIGHT CHARACTERISTICS OF TRIPLETS - A STUDY FROM THE DUTCH TWIN REGISTER

From 112 triplet sets, born in The Netherlands from the end of 1986 to the beginning of 1991 and registered in the Dutch Twin Register, seve ral details such as birth weight, gestational age, zygosity, and etiol ogy were assessed by questionnaire, which was filled out by the mother . For 33 triplet sets, zygosity was also assessed by blood typing. Mat ernal smoking during pregnancy was also noted. Results show a very str ong increase in number of triplets caused by artificial fertility enha ncing techniques and consequently a shift in the relative contribution of zygosity types to the total number of triplets. Birth weight is pr edominantly influenced by gestational age. Other effects on birth weig ht are controlled for possible confounding with gestational age. First born triplets weigh more than later born triplets; boys weigh more th an girls; nearly 25% of all individual triplets weigh less than 1500 g , i.e. belong to the category very low birth weight (VLBW); regular ma ternal smoking produces a 14% birth weight reduction; ovulation induct ion seems to decrease the sex ratio, i.e. hormonal treatment with ovul ation inducing substances increases the probability of female offsprin g.