Twin pregnancies in Cardiff and Vale of Glamorgan over four decades: natural twinning is on the increase

Objective

To assess trends in twinning over four decades using a population-based registry.

Design

Ecological study to conduct trend analysis of twin pregnancies in a geographically defined area over 40 years.

Setting

All pregnancies in the Cardiff and Vale of Glamorgan area of South Wales from 1965 to 2004, as recorded in the Cardiff Birth Survey (CBS) database.

Methods

Trends of the incidence of all twin pregnancies (≥18 weeks of gestation) were calculated in 5-year increments, beginning with 1965–1969 and ending in 2000–2004. Natural twinning rates could only be calculated for the terminal five time periods (i.e., 1980–1984, 1985–1989, 1990–1994, 1995–1999, and 2000–2004), when information regarding non-spontaneous (iatrogenic) twinning was first collected in the database. All results were adjusted for maternal age.

Results

The total twinning rate was 13.1 per 1000 pregnancies in the 1st time period (1965–1969). Subsequently, there was a gradual reduction in twinning, reaching a nadir of 10.3 per 1000 for the time period 1980–1985 (Z = 3.15, P value < 0.001). This was followed by a gradual increase in twinning, reaching a maximum of 15.7 per 1000 for both 1995–1999 and 2000–2004 (Z = −5.18, P value < 0.0001). After exclusion of the cases of iatrogenic pregnancies, the natural twinning rate showed a continuous and gradual increase from 10 per 1000 spontaneous pregnancies in 1980–1984 to 13.3 per 1000 in 2000–2004 (Z = −5.08, P value < 0.0001).

Conclusion

The data showed a gradual, continuous increase in natural twinning rates over the last two decades. Such an increase cannot be attributed to the rise in maternal age alone.