The Probability of Becoming Pregnant with Twins

Non-Identical Twins : image by Nate Davis (EtanSivad)

Non-Identical Twins : image by Nate Davis (EtanSivad)

Congratulations, you’re pregnant! What is the probability that you are carrying twins?

Or perhaps you’re not yet pregnant, but you’re wondering “How do I get pregnant with twins”?

Maybe twins run in your family, and you’re afraid you might be next…

Let’s consider the chances of a pregnancy with twins, and what might affect those odds.

Probability of Twins: Two Types of Twins Carry Different Chances

There are two types of twins pregnancy: monozygotic twins and dizygotic twins.


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  • Monozygotic twins develop from a single ovum, or egg, that splits into separate individuals shortly after conception.
  • Dizygotic twins began as separate eggs that were fertilized by different sperm cells. (These words come from the Greek: “mono” for “one”; “di” for “two”; and “zygote” for “joined”, which refers to the united egg and sperm before it develops into an embryo).

The probability of having monozygotic twins is 0.4% (or “4 in 1000 births”). This rate seems to hold regardless of other factors. However, the probability of having dizygotic twins varies depending on the mother’s racial origins.

  • Black mothers have 10-40 dizygotic twins per 1000 births.
  • Caucasians have 7-10 dizygotic twins per 1000 births.
  • Asians only 3 dizygotic twins per 1000 births.

(Those percentages are 1% to 4%; 0.7% to 1%; and 0.3% respectively).

Another factor for having dizygotic twins is maternal age. Until about age 40, the chance of having dizygotic twins increases with age.

Yet another, and rather well-known, factor is assisted fertilization. Some fertility drugs promote an increased probability of having twins.

The Incidence of Twins in the United States

Here we discuss the average rate of multiple births in the United States, blended across all racial backgrounds, maternal ages, fertility treatments and any other factors.

Dr. Fletcher stated in his Multiple Births article that in the period “2004-2006, the prevalence of twin deliveries in the United States has remained stable at approximately 32 per 1000 births“. He noted that this rate of 3.2% was higher than the rate of twin pregnancy in 1970.

Martin, et al, agree that the rate had been stable from at least 1915 until the 1970s. In Three Decades of Twin Births in the United States, 1980–2009, the authors report the rate was about 2%, and has risen to about 3.4% in 2009.

These statistics blend the various racial probabilities and medical situations into one simple probability percentage number. On average, an American woman has about a 3.3% chance of becoming pregnant with twins.

How do You get Pregnant with Twins?

Twin Girls : image by Ruth L

If you take fertility drugs or are in your late thirties, you have a higher chance of having twins. Image by Ruth L

We’ve already seen the basic mechanisms. Either you had two eggs, and each was fertilized; or your one fertilized egg split into two individuals during its first days of development.

Can you influence your chances of getting pregnant with twins?

If you become pregnant in your late thirties, you increase your chances of a twin pregnancy slightly.

Taking fertility drugs is a more “reliable” way to influence the odds. But note that Derman and Adashi include the likelihood of multiple births in their Adverse Effects of Fertility Drugs. In 1994, they cited the fertility drug gonadotrophin for this side effect.

Derman and Adashi reported a range of potential adverse effects from the fertility drugs available at that time. If you are considering any type of medical treatment, do ask your medical practitioner about potential side effects, including the probability of becoming pregnant with twins.

Increasing or Reducing Your Chance of Having Twins

Whether you want twins or not, keep an eye on the factors that affect multiple pregnancies. If you want to avoid having twins, don’t wait till later in life to get pregnant, and avoid fertility drugs. If you like the idea of having more babies to love, then talk to your doctor about ways to increase your chances of a twin pregnancy.

Resources

Twin Boys : image credit Lindsay Mgbor/Department for International Development (UK)

Will you have twins? Your chances of a multiple birth increase with age. Image credit Lindsay Mgbor/Department for International Development (UK)

Derman, Seth G. and Dr. Adashi, Eli Y. Adverse Effects of Fertility Drugs. (1994). University of Maryland. Referenced August 1, 2013.

Fletcher, Garth E. (MD). Multiple Births (Epidemiology section). (2007).  MedScape. Referenced August 1, 2013.

Martin, Joyce A, et al. Three Decades of Twin Births in the United States, 1980–2009. (2012). CDC. PDF referenced August 1, 2013.

© Copyright 2013 Mike DeHaan: Math, All rights Reserved. Written For: Decoded Pregnancy
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