Monitoring of trends in the incidence of multiple births is important because such births pose a risk to the health of the mother and fetuses. Trends in twinning also have a defined role in epidemiological studies analyzing interactions of environment and genetics, and may be useful to monitor population fertility. In the 1930s and 1940s, there was a marked reduction in the incidence of multiple pregnancy rates in the United States, the United Kingdom and other European countries. Following the nadir reached in the 1970s, there has been a continuous increase. Little data is available for natural twining rates.
To assess trends in twinning over a 40-year period from 1965 to 2004 in a geographically defined area, the authors examined pregnancy data from a population-based database, the Cardiff Birth Survey (CBS). They divided the data into eight 5-year periods (1965–1969, 1970–1974, 1975–1979, 1980–1984, 1985–1989, 1990–1994, 1995–1999, and 2000–2004). Because infertility treatment data regarding nonspontaneous (iatrogenic) twinning was not collected by the CBS before 1980, the trend data for the early three periods (1965–1969, 1970–1974, and 1975–1979) reflected the total twinning rates, not natural twinning rates. Data missing from the CBS database for the years 1985 through 1989 did not appear to affect the findings. All results were adjusted for maternal age.
In this retrospective study, the total age-adjusted twinning rate per thousand pregnancies gradually declined from 13.5 in the 1965 to 1969 period to a nadir of 10.3 in the 1980 to 1984 period (P < .001). For the next 4 periods (1985–2004), the rate increased gradually reaching 15.7 from 2000 to 2004 (P < .0001). The natural twinning rate gradually increased from 10 per 1000 spontaneous pregnancies from 1980 to 1984 to 13.3 per 1000 from 2000 to 2004 (P < .0001).
The authors conclude from these findings that there has been a gradual, steady rise in natural twinning rates over the last 2 decades. Possible contributing factors include increases in maternal age or preconception use of multivitamins, folic acid and/or other nutritional factors, although these factors alone cannot account for the entire twinning increase.