A guide for anyone TTC and wondering how to increase the chances of having twins.
By OBGYN and fertility expert Dr. Kenosha Gleaton
Dreaming about “Twin 1” and “Twin 2” onesies, a split nursery, or a double stroller? Twin births have nearly doubled over the last 40 years, which coincides with an increase in fertility treatments. Let’s break down why twins exist, what role genetics play, and why carrying multiples can be risky.
How twins occur
There are two types of twins, identical and fraternal. Identical twins occur when a single embryo divides into two embryos. Each embryo is genetically identical and will be the same sex.
Fraternal (non-identical) twins occur when two separate eggs are each fertilized by two separate sperm. The two embryos that result are not genetically identical and can therefore be either the same or different sex. Fraternal twins are far more common than identical twins, especially when a twin pregnancy results from assisted reproduction.
According to data from 2019, there were a total of 120,291 twin births in the United States, making up about 3% of the total registered births (3.75 million). The twin birth rate was 32.1 per 1,000 live births. Triplet births (3,136 total) or higher (150 total) occurred at a rate of 87.7 per 100,000 live births.
How triplets (and more) occur
While they’re far more rare, triplets, quadruplets and more also occur from time to time. The biology is the same — an egg splits, causing identical twins/triplets/potentially quadruplets. That’s right — identical quadruplets are a possibility, and have occurred, but the chances are about one in 10+ million. In a multiple pregnancy higher than twins, there is the possibility of identical embryos, fraternal, or a combination of both. The most likely scenario is when the body releases multiple eggs, and more than one is fertilized, resulting in fraternal multiples. This is likely due to an increased maternal age as well as the use of assisted reproduction.
There are no genetic links that we know of to explain identical twin births, but a family history of fraternal twins does increase your chances of carrying fraternal twins. Family history on the mother’s side can mean a higher chance of having twins than history on the father’s side. Research shows fraternal twin women give birth to twins at a rate of one per 60 births, compared to the general population having twins in about one per 250 pregnancies. Meanwhile, fraternal twins father twins at a rate of one per 125 births.
Age, weight, and height
- Age: Women over 30 are more likely to conceive twins than those under 30. As women age, hormones such as follicle-stimulating hormone (FSH) rise. FSH is responsible for the development of eggs, and higher FSH levels means multiple eggs may be released, resulting in a multiple pregnancy.
- Weight: Overweight women are more likely to conceive twins than those of an average weight. Excess body fat leads to an increase in estrogen, which can overstimulate ovaries and cause the release of more than one egg during ovulation. However, I do not recommend gaining weight in hopes of having twins. The likelihood is only increased slightly, while the negative health impacts are often significant and long lasting.
- Height: Taller women are more likely to have twins. A study found those averaging around 5’5” were more likely to conceive twins than those averaging 5’4”. There’s no clear cause, but one theory suggests that better nutrition is actually a factor.
Medications that increase the chances of having twins
To be clear, there’s no medication you can take to magically get pregnant with multiples, but there are noticeable associations. Medications and hormones given during fertility treatments can have an impact on hyperstimulation of the ovaries. The ovaries are triggered by hormones to release an egg every month for ovulation. When fertility medications trigger or create the necessary hormones, the ovaries can be hyperstimulated, potentially releasing numerous eggs in one cycle, and resulting in a multiple pregnancy. Some medications that may increase the chances of hyperstimulation include:
- Clomiphene: An estrogen modulator that requires a prescription, clomiphene is commonly given to women struggling with infertility. Of pregnant women that used clomiphene, about 5% to 12% bear twins, and under 1% bear triplets or more.
- Gonadotropins: Injectable medications used during fertility treatments, gonadotropins contain hormones like FSH and luteinizing hormone (LH). The use of gonadotropins to achieve pregnancy has resulted in approximately 30% of multiple pregnancies. The majority are twins, but up to 5% are triplets or greater.
A common cause of multiple pregnancies is fertility treatments. This can include IVF, IUI, injectables, and other fertility medications.
- IVF: While not so common anymore, in the early days of ART (assisted reproductive technology) around 3-4 embryos would be placed in the uterus at one time, in the hopes of a single baby. With increased technology and experience with IVF, standard of care is for fewer embryos to be transferred, but the twin rate is still high.
- IUI: The transfer of semen into the uterus, commonly alongside the use of fertility medications. While IUI by itself doesn’t necessarily increase your chances of having twins, increased hormones due to fertility drugs can.
Having multiples (twins, triplets, or more) is riskier for mother and baby
While having twins may sound fun, it’s important to understand the inherent risks of carrying . twins, triplets, or more. There are risks associated with multiples, including a higher chance of premature birth, prenatal death, intrauterine growth restriction, and risks to maternal health such as preeclampsia, diabetes, and birth complications.
- There are two types of twins: fraternal and identical. Fraternal twins are more common than identical twins.
- A woman is more likely to conceive twins if she or her partner has a family history of fraternal twins, if she’s over the age of 30, overweight, and taller than average. It is important to note that some factors such as age and weight can increase other risk factors as well.
- Fertility medications such as gonadotropins and clomiphene can increase hormones like FSH and LH, which can cause multiple eggs to be released during ovulation.
- Fertility treatments like IVF and IUI can also increase the chance of conceiving twins.