Do twins run in your family? Wondering if your odds are higher than most? Take this Chances of Having Twins Quiz!
How Common Are Twin Pregnancies?
According to data collected by the Centers for Disease Control, there were 138,660 twins born in the United States in 2008. That's 32.6 per 1,000 live births, or put another way, about 3.26% of live births.
There were 5,877 triplet births, 345 quadruplet births, and 46 quintuplet or higher order births.
These numbers include naturally occurring multiples, along with those conceived with fertility treatment.
The rate of multiple births has increased over the years. For example, more twins were born to women ages 45 to 49 in 1997 than during the entire 1980s.
The increase in twin births is partially because more women are having children after 30. (More on this below.) "Older" moms make up one-third of the increased twinning rate.
The rest of the increase is due to fertility drug and treatment use.
Chances of Having Twins with Fertility Treatments
Fertility treatments that boost ovulation can lead to twins, triplets, or higher order multiples. Not all treatments for infertility come with this increased risk, but most do.
Fertility drugs and treatments that may lead to twins include...
- Gonadotropins (also known as injectables) like Gonal-F and Follistim
- IUI treatment, when used along with fertility drugs
- IVF treatment
Clomid and Femera have the lowest rate of twins, ranging from 5 to 12%. The rate of triplets and higher order multiples is under 1%.
Gonadotropins, whether used with or without IUI treatment, have the highest rate of twins. According to some studies, up to 30% of pregnancies conceived with gonadotropins lead to multiples. Most of these pregnancies are twin pregnancies, but up to 5% are triplet or higher order pregnancies.
Contrary to popular belief, IVF treatment is not the main source of triplet and higher order pregnancies. The latest data collected by the CDC indicate that the rate of triplets in 2009 was about 3.6% for women aged 35 to 40, and lower for women under than 35 or older than 40.
IVF twins are relatively common, with the twin rate highest for women younger than 35, at 33.4% in 2009. The IVF twin rate is lower for women over age 35, likely due to the overall decreased success rate as a woman ages.
Causes of Twins Besides Fertility Treatments
Fertility treatments are not the only cause for twins. Other factors that increase your chances of getting pregnant with multiples include...
Age -- Women over 30 have an increased risk of twins. This is because the hormone FSH rises as a woman gets older. FSH, or follicle stimulating hormone, is responsible for the development of eggs in the ovaries before they are released. Higher levels of FSH are needed as a woman ages, because the eggs require more stimulation to grow than in a younger woman.
This is somewhat ironic, given that increased FSH is also due to lowering fertility. But sometimes, the follicles overreact to the higher FSH levels, and two or more eggs are released, resulting in a twin pregnancy.
Family history -- If you have fraternal twins (non-identical) in your family, your chances of conceiving twins rises. A family history of identical twins does not, however, increase your risk of twins.
Perhaps surprisingly, a family history of twins increases your chances even more so if history is found on both the female and male side. A history of twins on the female side of the family indicates a higher likelihood of ovulating more than one egg per cycle, and a history of fraternal twins on the male side indicates a higher likelihood of the man producing enough sperm to fertilize more than one egg.
Weight -- Women who are obese, with a BMI over 30, are more likely to conceive twins than women with a healthier BMI. This is another ironic situation, since overweight women are also more likely to have difficulty conceiving.
The extra fat leads to your body producing increased amounts of estrogen. The increased levels of estrogen then may lead to over stimulation of the ovaries, and instead of releasing just one egg at ovulation, the ovaries may release two or more.
Height -- Women who are taller than average have an increased risk of conceiving twins. One study found that women averaging 164.8 cm in height (about 5' 4.8") were more likely to conceive twins than women averaging 161.8 cm (about 5' 3.7"). Why this happens isn't clear, but one theory is that better nutrition (which may lead to more height) is partially behind the increased rate of twins.
Number of children -- Twins are more common in women who have carried many pregnancies and have large families.
Race -- African-Americans are more likely to conceive twins than Caucasian women. Asian women are the least likely to conceive twins.
Breastfeeding -- Women who conceive while breastfeeding are more likely to conceive twins than women who are not. It's true that breastfeeding can also suppress fertility and prevent pregnancy, specifically during a baby's first six months if the baby is exclusively breastfed.
However, it is possible to get pregnant when breastfeeding -- and with twins! One study found the rate of twins to be 11.4% among breastfeeding women, as compared to just 1.1% in non-breastfeeding women.
Diet -- While research is still on going, some studies have found that women who eat a lot of dairy products are more likely to conceive twins. One theory is that the growth hormones given to cows affect the hormone levels in humans.
Chances of Having Identical Twins
In the general population, identical twin pregnancies occur 0.45% of the time, or 1 in 250 births.
While most multiple pregnancies conceived with fertility treatments are fraternal twins, the use of fertility treatment does increase your risk of having identical twins. According to one study, identical twins made up 0.95% of the pregnancies conceived with treatment. That's double the general population's risk.
It's unclear why fertility treatment leads to more identical twins. One theory is that the culture embryos are placed in during IVF increases the risk of identical twinning. Another theory is that treatments using gonadotropins lead to the increased risk of identical twins.
Bottom Line on Your Chances of Having Twins
Your chances of having twins will depend not just on your use of fertility drugs, but also your family history, race, age, and many other factors. These factors do work together. In other words, a tall woman with a family history of fraternal twins is more likely to conceive twins during fertility treatments than a short woman without any family history of twins.
Your chances of conceiving twins will also be affected by your particular cause of infertility. A young woman with healthier eggs is more likely to conceive twins than a woman over 40, whose egg quality is poor.
The twin and multiple rates also vary from fertility clinic to clinic. Twin rates differ based on how carefully they track ovulation stimulation during fertility drug use and how many embryos they transfer during IVF. Even if your doctor decides to try single embryo transfer, you may still conceive twins -- identical ones!
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