How Common Are Twins?

How Common Are Twins Naturally?

Naturally (without the aid of fertility drugs) twin conceptions occur in about 1 in every 90 pregnancies in white women. Triplets occur in about 1 in 10,000 and quadruplets in about 1 in half a million pregnancies. The natural occurrence of higher multiples, up to octuplets (8 babies), has been recorded. The spontaneous (natural) incidence of twins is greater in black women, reaching 1 in 30 in some West African tribes, and lower in the Mongol race (1 in 150). A tendency to multiple pregnancy is inherited, and it occurs more frequently in certain families. A woman who has given birth to twins is said to be 10 times more likely to have a multiple conception in a subsequent pregnancy than a woman who has not previously had twins. Also, tall women are more likely to have twins than small women.

Related resource: Pregnant with twins, and Twin DNA testing.

Do Twins Skip A Generation?

No, not necessarily. While twins can run in families, there is no evidence that they skip a generation. Some time ago scientists identified a gene that can be inherited and predisposes a woman to releasing more than one egg in a single menstrual cycle (hyperovulation). When both eggs are fertilized the resulting twins are fraternal (not identical). For this reason fraternal twins often run in families. So why the myth that twins skip a generation? It’s probably just an illusion because men who inherit the gene from their mothers are unaffected by it - they can’t become pregnant. But if they pass it onto a daughter and she conceives with twins, it can look like it skipped a generation.

Are Older Women More Likely To Have Twins?

Yes, the incidence of twin pregnancies rises slightly with increasing maternal age up to 40. This is because as a woman approaches perimenopause, hormone changes make it more likely that her body will release more than one egg at a time. How does this happen? Women over the age of 35 produce more follicle stimulating hormone (FSH) than younger women. Increasing levels of this hormone ironically are a sign of declining fertility. But as FSH is the hormone that causes an egg to mature for ovulation every month, extra FSH can cause more than one egg to mature and release. So while older women are less likely to get pregnant, if they do, they are more likely to have twins.

What Are The Risks Of Twins When Taking Fertility Drugs?

Fertility drugs are medications which are taken to stimulate ovulation in women who have problems becoming pregnant. They work by increasing the level of certain hormones in your body so that you release one or more eggs a month (ovulation). According to statistics by the Centers for Disease Control and Prevention (CDC) ovulation treatments account for nearly 25 percent of all multiple births in the United States. As the number of women turning to fertility treatment increases, so too are the number of twins and triplets being born. It is estimated that about 190,000 children are born every year in the US by ovulation induction. The risk is slighter with clomiphene (Clomid and Serophene) but considerably higher with gonadotrophins (Pergonal, Repronex, Menogon, Fertinex, Follistim, Gonal F, Purego, Pregnyl, Novarel, Profasi and Ovidrel).
If you undergo IVF (in vitro fertilization) - the next line of treatment after fertility drugs - your risk of having more than one baby increases by between 20 and 40 percent.

Related Topics
Endocrine system: What are hormones and what they do in the body.
Infertility guide: Tests, treatments and advice for women having difficulties conceiving.
Egg donation: A guide to donor eggs.

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