Deben Doubles Twin Club

How common are multiples?

In 2008 there were 701,297 maternities in England & Wales. Of which 98.5% were singletons and 1.5% multiple. Of the 10,855 multiple maternities 98% were twins and 2% triplets or more.

According to the Human Fertilisation and Embyology Authority (HFEA) – the UK’s independent regulator overseeing the use of embryos in fertility treatment and research – about one in 80 births resulting from natural conception is a multiple and one in four births after IVF (in vitro fertilisation) result in either twins or triplets.[1]

Is the number of twins and triplets increasing?

The last 20 years has seen an upward trend in the number of multiple births, although since 1997 the yearly increase has slowed somewhat compared to the previous 10 years as the number of triplet births has reduced following guidelines (in 1997) that a maximum of two embryos should be replaced in IVF treatments.

In future it’s likely that fertility clinics will only be allowed to transfer a single embryo in some cases, in order to bring down the high rate of IVF-related twin births.
Another reason for the recent increase has been the result of more couples having babies at an older age. Between 1996 and 2006 alone, there was a 182% increase in multiple pregnancies in women aged 35 or over.[2]

2008 population data shows that in 62% of all multiple maternities the mother was over 30 years old, compared to 47% of singletons, 6% compared to 4% were over 40 years old.[3]

One reason for this increase is that older women are more likely to conceive a multiple pregnancy naturally than younger women. As you get older, you naturally produce more ovulation-stimulating hormones, which could trigger your ovaries to release several eggs each month. Another, and probably more significant, factor is that older couples are more likely to use fertility drugs and treatments such as IVF to assist conception.

What other factors increase the chances of having multiples?

Heredity is another important factor. Non-identical (fraternal) twins tend to run in families, usually – but not always – on the mother’s side. If twins run in your mother’s family, you’re more likely to have a set yourself. They also occur more often in certain ethnic groups. Identical twins, which are the result of a single fertilised egg splitting, don’t tend to run in families.

Size matters, too. A US study showed there was a significant increase in non-identical twin births to mothers who had a BMI of 30 or above, or who were in the top 25th percentile for height.[4]
Identical & non-identical
About one third of twins are identical or monozygotic twins (when one fertilised egg divides in half). Two thirds are non-identical or dizygotic twins (when two eggs are fertilised by different sperm).

The chance of any woman having identical twins is around one in 250.[5] But your chance of having non-identical twins varies according to whether they run in your family or not.[6]
The rate of identical twins is also increased after IVF for reasons that aren’t fully understood, but the vast majority of IVF twins are non-identical.[7]

[2] Office of National Statistics, 2007.

[3] Office of National Statistics, 2008.

[4] Reddy, 2005

[5] Baker, 2006

[6] Baker, 2006

[7] HFEA 2006

[8] The Multiple Births Foundation www.multiplebirths.org.uk