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  • Title: Multiple pregnancy and fetal abnormalities in association with oral contraceptive usage.
    Author: Macourt DC, Stewart P, Zaki M.
    Journal: Aust N Z J Obstet Gynaecol; 1982 Feb; 22(1):25-8. PubMed ID: 6954940.
    Abstract:
    1,000 multiple births were examined in relation to maternal exposure to an oral contraceptive before conception. There was a significantly higher rate of monozygotic twinning among pregnancies which took place within 6 months of cessation of the contraceptive. In the monozygotic group there was a significant increase in the incidence of congenital abnormalities (P less than 0.01) when conception occurred within 3 months of cessation of the contraceptive. Retrospective questionnaires were completed by mothers of multiple births in order to examine oral contraceptive (OC) usage in association with the incidence of multiple births, zygosity, and the presence of congenital abnormalities. The questionnaire was distributed nationwide to 2000 members of the Australian Multiple Birth Association seeking data in relation to biological aspects of twinning, including maternal and paternal ages, parity, sex and ages of all children in the family, and diseases, deformities and illnesses including stillbirths and deceased members of the family. Parents were requested to identify whether times were the same or of different sex and whether they considered them to be identical or not identical. In relation to each set of twins in the family, the mother was requested to supply retrospective details about OC use before conception and the elapsed interval between cessation of use and conception. At the time of this report, 1006 responses had been received; 1000 were included in the analysis. National statistics in Australia were examined for all births and both monozygous and dizygous multiple births from 1950-1972. The statistics demonstrated a marked decrease in the percentage of dizygotic twin births in Australia and also an increase in the proportion of monozygotic twins. A regression analysis on this data was highly significant for both the increase in monozygotic twinning rates and the decrease in dizygotic twinning rates. The ratios of monozygotic to dizygotic twins (MZ:DZ ratio) in relation to cessation of use of OCs in 3-monthly intervals was compared with that found in non-OC users. The MZ:DZ ratio of 128:273 (32%) among non-OC users was as expected. In OC users, there was a higher than expected rate of monozygotic twinning among conceptions which occurred within 6 months of cessation of OC use. This was particularly noticeable in the 1st month, where the MZ:DZ ratio was 40:42, almost 1:1. After 6 month cessation of OCs, the rate was more consistent with the rate expected. The proportion of congenital abnormalities among dizygotic twins appeared to be fairly constant both in the OC and non-OC user groups; the minor variations were not statistically significant. In the monozygotic group, the incidence of abnormalities was increased in the group where the mother had conceived within 6 months of cessation of the OC. If the child was conceived in the first 3 months, and particularly in the 1st month after stopping OC use, the increase in the rate of congenital abnormalities was highly significant.
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