a1 Epidemiological Centre, National Board of Health, Stockholm, Sweden.
a2 Tornblad Institute, Lund, Sweden. email@example.com.
a3 Department of Obstetrics and Gynaecology, University Hospital, Lund, Sweden.
Folic acid is recommended to reduce the risk of neural tube defects and other congenital malformations. Data from the Swedish Medical Birth Registry were used to study frequency of twinning in women who in early pregnancy reported the use of folic acid. Women (n = 2,569) who in early pregnancy reported the use of folic acid had an increased rate of twin deliveries after consideration of maternal age and of length of involuntary childlessness, both variables being significant confounders. The effect of folic acid was seen also in women who did not report involuntary childlessness. A similar but not statistically significant trend was seen after use of multivitamins without simultaneous use of folic acid tablets (n = 1,979). The increased risk seems to be limited to dizygotic twinning (relative risk = 2.13, 95%CI 1.64–2.74). If this association is causal, wide-spread supplementation with folic acid may represent a hazard larger than the postulated beneficial effect on neural tube defects, at least in low-risk areas.
(Received January 14 2001)
(Accepted February 16 2001)
c1 Address for correspondence: Professor Bengt Källén, Tornblad Institute, Biskopsgatan 7, S-223 62 Lund, Sweden.