ANNALS OF HUMAN BIOLOGY,
1979,
VOL.
6, NO. 5,
481-483
Maternal age, dizygotic twinning rates and age at menopause W. H. JAMES Department of Human Genetics and Biometry, University College London
Ann Hum Biol Downloaded from informahealthcare.com by Nyu Medical Center on 12/10/14 For personal use only.
Received 19 February 1979; revised 15 May 1979
Summary. The dizygotic (DZ) twinning rate declines after about maternal age 38 in Caucasian populations. Bulmer (1970) offered the explanation that waning ovarian function causes a decline in rates of double ovulation in originally twin-prone women. The present note suggests that some small part of the decline is caused by the earlier cessation of ovulation altogether in these women (presumably also as a result of waning ovarian function).
It is well known that the dizygotic twinning rate increases from a low value following menarche to a peak at around age 37 or 38 (in Caucasian populations) and then declines sharply to a value of near zero at the menopause. Bulmer (1970) illustrates the point for Italian births 1949-54. He shows that this curious regression exists even when parity is controlled, and that it is more extreme when maternal age is categorized in single years rather than in 5-year periods. The cause of this regression is not well understood. It is thought that double ovulation is under hormonal control, and indeed it is clear that the effects of gonadotrophin and clomiphene are dose-related. N ylander (1973) has noted supporting evidence in the variations in FSH concentrations between women who have borne singletons only, one pair of twins, and two pairs of twins. And it seems clear that hormone levels increase with age within individual women: FSH levels rise during the reproductive years, and after menopause, gonadotrophin levels remain high (Reyes, Winter and Faiman 1976). Since these levels do not decline towards the end of the reproductive years (from age 37 on, say), an explanation is required for the decline in dizygotic twinning after that age. Bulmer (1970; p. 81) suggests that waning ovarian function is the cause. Lazar (1976) offers the suggestion that increasing spontaneous abortion rates may be responsible. This latter suggestion seems inadequate because spontaneous abortion rates do not increase rapidly enough with maternal age to be able to account for more than a slight part of the effect (James 1974, 1977). The point of this note is to offer an alternative (or supplementary) explanation. It is that DZ-twin-prone women reach menopause earlier than other women on the average (Wyshak 1978). In this author's data, the mean age at natural menopause of women who had had opposite-sexed twins (47.99) was less (P
1979,
VOL.
6, NO. 5,
481-483
Maternal age, dizygotic twinning rates and age at menopause W. H. JAMES Department of Human Genetics and Biometry, University College London
Ann Hum Biol Downloaded from informahealthcare.com by Nyu Medical Center on 12/10/14 For personal use only.
Received 19 February 1979; revised 15 May 1979
Summary. The dizygotic (DZ) twinning rate declines after about maternal age 38 in Caucasian populations. Bulmer (1970) offered the explanation that waning ovarian function causes a decline in rates of double ovulation in originally twin-prone women. The present note suggests that some small part of the decline is caused by the earlier cessation of ovulation altogether in these women (presumably also as a result of waning ovarian function).
It is well known that the dizygotic twinning rate increases from a low value following menarche to a peak at around age 37 or 38 (in Caucasian populations) and then declines sharply to a value of near zero at the menopause. Bulmer (1970) illustrates the point for Italian births 1949-54. He shows that this curious regression exists even when parity is controlled, and that it is more extreme when maternal age is categorized in single years rather than in 5-year periods. The cause of this regression is not well understood. It is thought that double ovulation is under hormonal control, and indeed it is clear that the effects of gonadotrophin and clomiphene are dose-related. N ylander (1973) has noted supporting evidence in the variations in FSH concentrations between women who have borne singletons only, one pair of twins, and two pairs of twins. And it seems clear that hormone levels increase with age within individual women: FSH levels rise during the reproductive years, and after menopause, gonadotrophin levels remain high (Reyes, Winter and Faiman 1976). Since these levels do not decline towards the end of the reproductive years (from age 37 on, say), an explanation is required for the decline in dizygotic twinning after that age. Bulmer (1970; p. 81) suggests that waning ovarian function is the cause. Lazar (1976) offers the suggestion that increasing spontaneous abortion rates may be responsible. This latter suggestion seems inadequate because spontaneous abortion rates do not increase rapidly enough with maternal age to be able to account for more than a slight part of the effect (James 1974, 1977). The point of this note is to offer an alternative (or supplementary) explanation. It is that DZ-twin-prone women reach menopause earlier than other women on the average (Wyshak 1978). In this author's data, the mean age at natural menopause of women who had had opposite-sexed twins (47.99) was less (P
