How do twins form in the womb? 

If you’re striving for the two-in-one baby deal, let’s learn first—how do twins come to be?

Essentially, conceiving twins requires an abnormality to occur during fertilization or during the formation of an embryo (the very start of your little one). Let’s explore the two types of twins and how they each form in the womb.

Identical Twins 

When people think of “twins,” they usually imagine identical twins.

Identical twins are also known as monozygotic twins (“mono” meaning “one”, “zygotic” meaning “egg”). These types of twins form from the same egg and therefore share the exact same DNA. Accounting for nearly 15% of all twins, identical twins form at conception through the following process:

1. Fertilization – A single sperm cell fertilizes a single ovum (or egg) in the fallopian tubes, creating a single-cell zygote.
2. Cell growth – The zygote travels down into the uterus, rapidly splitting into multiple cells to form a blastocyst.
3. Balstocyst division – Once in the uterus, the blastocyst abnormally splits into two. These equal parts will form two independent embryos—the start of all identical twins.

Fraternal Twins 

Have you ever mistaken twins for siblings? You were probably looking at a pair of fraternal twins—a.k.a, non-identical twins. Fraternal twins only share half of their DNA, just like typical siblings do. This is because fraternal twins grow from different sperm cells and egg cells (just like sequential siblings), earning the name dizygotic (“two eggs”). 

Fraternal twins are actually more common than identical twins—over three-quarters of all twins are fraternal. Inside the womb, fraternal twins form in this process:

1. Hyperovulation – Hyperovulation occurs when an ovary releases not one fertilized egg but two eggs during ovulation.
2. Double fertilization – Then, separate sperm cells fertilize each egg. Genetically, this creates “sibling” zygotes that happen to be in the same womb.
3. Embryo growth – Both zygotes travel to the uterus, growing into distinct embryos. At this point, you’re carrying multiple babies at the same time. 

“Twinning” Facts 

Whether identical or fraternal, twins carry a few distinct genetic and medical traits. If you’re hoping for twins, check out these important facts connected to twin pregnancies, births, and children:

• Gestation – The normal gestation period is 40 weeks, but twin gestation periods usually take just 38 weeks. Scientists believe twins may heighten conditions that cause preterm deliveries, from preeclampsia to placental dysfunction.
• Morning sickness – After conception, the human chorionic gonadotropin hormone (hCG) rises to tell your body it’s pregnant—and potentially triggers morning sickness. Carrying more than one baby can boost hCG levels and, in turn, increase this less-than-pleasant pregnancy symptom.
• Pregnancy & birth risks – Although twins bring double the happiness, they also bring some health risks. Women pregnant with twins have higher rates of gestational hypertension, anemia, and even miscarriage. Twins also experience twice the risk ​​of congenital birth defects, such as a neural tube, lung, and heart abnormalities.

Natural Causes of Twins 

Most natural causes for twins are genetic. However, a few lifestyle choices can influence your likelihood of twin pregnancy. Between nature and lifestyle, see if you meet the following factors for twin pregnancies. 

Natural factors: 

• Age – The older you are, the more likely you will become pregnant with twins (at least fraternal ones). This is because women in their 30s and 40s have higher levels of follicle-stimulating hormone, which can accidentally trigger hyperovulation (when an ovary releases more than one egg at the same time—the potential start of fraternal twins).
• Genetic history – It really does run in the family. If mom is a fraternal twin, she has a one in 60 chance of conceiving twins (fraternal or identical). Similarly, male fraternal twins have a 1 in 125 chance of conceiving twins. For comparison, non-twin parents have a 1 in 250 chance of giving birth to twins.  
• Race – According to the CDC, black women in the US have the highest twin birth rate. Asian and Hispanic women have the lowest rates. 
• Weight and height – Women with more body fat have more estrogen, which can trigger hyperovulation. This leads to higher fraternal twin conception rates for an overweight woman. Similarly, tallness gives a twin advantage. Research has found that taller women have a greater chance of conceiving twins. This may be due to better nutritional intake during childhood, leading to a stronger body with more regular ovulation cycles.  

Lifestyle factors: 

• Previous births – The bigger your nest, the faster it may grow. Scientists are unsure why, but mothers with more past pregnancies have an increased chance of conceiving twins—especially if they’re older in age. Women between the ages of 35 and 40 with four or more previous births are three times more likely to have twins than childless women under 20. 
• Breastfeeding – If you’re going back-to-back with pregnancies, you might get a double-surprise. One study found that breastfeeding women are 9 times more likely to conceive twins if they get pregnant.
• Nutrition – Surprisingly, a study found that women who eat animal products and dairy are five times more likely to conceive twins. Researchers believe animal products increase insulin-like growth factor (IGF), which can stimulate hyperovulation. 

Medical or Artificial Causes of Twins 

Due to hormonal interference, many fertility treatments raise the likelihood of multiple-child births. In particular, these fertility treatments may increase your odds of having multiple babies at once:

• Clomid (clomiphene) – An oral drug, Clomid boosts levels of follicle-stimulating hormone and luteinizing hormone to stimulate ovulation. 
• Femara (letrozole) – By inhibiting estrogen, Femara causes your body to boost follicle-stimulating hormone and release an egg for ovulation. 
• Gonadotropins – These injectables mimic follicle-stimulating hormones, causing your ovaries to release multiple eggs. 
• Assisted reproductive technology – From IVF to frozen embryo transfer, assisted reproductive technology includes any treatments that directly assist with egg fertilization and implantation.

However, it’s important to note here that no fertility doctor will prescribe any fertility medication for the sake of creating twins—it’s just too risky for your health and may even cause a pregnancy complication. 

What are my odds of having twins? 

Predicting the arrival of one baby is hard enough. Add another, and you get a complicated equation. 

Everyone has their own likelihood of conceiving two peas-in-a-pod, based on all of the natural and artificial factors listed above (and some luck). However, you can form an idea of your odds by referring to some statistics.

Overall Odds of Conceiving Twins 

The overall twin birth rate in the US has risen 70% over the last 42 years, but most of the increase is due to increased use of fertility treatments and higher average maternal age. 

In 2020, the CDC reported that multiple-child births accounted for 3.32% of all births in the US. 

• Twins – Overall, twins make up about 3.11% of total births in the US. The chances of having identical twins is even lower, sitting at just 0.45% of total births. 
• Triplets – In 2020, triplets made up just 2.34% of all multiple-child births. Without fertility treatment intervention, the odds of conceiving triplets are one out of 1,800 pregnancies.
• Quadruplets or higher – Anything above three children is a rarity. In 2020, only 137 mothers gave birth to four or more babies at once—pretty much a one-in-a-million chance.

Fertility Treatments and Odds of Conceiving Twins 

If you get pregnant through fertility treatment, your odds of having twins jump to 1 out of 3 pregnancies—that’s a major boost. Here’s how each common fertility treatment can affect your likelihood of seeing double:

• Clomid – About 5% to 12% of Clomid pregnancies result in twins. For triplets, the odds drop to less than 1%.
• Femara – Studies are split on Femara’s “twinning” effect. One study says Femara carries a 14.3% chance of multiples, while other research claims the likelihood of twins is just 3.4%. 
• Gonadotropins – A heavy hitter, gonadotropins have a near 30% multiple-child rate for mothers. Almost 95% of these multiple-child pregnancies will result in twins.
•  Assisted Reproductive Technology (ART) – From in-vitro fertilization to gamete intrafallopian transfer, ART raises your odds of a multiple-child pregnancy to 21.4%. Out of all multiple-child pregnancies in the US, about 12.5% used ART.

Why the baby boom with fertility treatments? Medical experts believe that increased estrogen and FSH from these treatments can trigger hyperovulation—the starting point for fraternal twins. However, scientists do not yet know why identical twin rates are higher for women who undergo these treatments.

Before you set out to conceive twins, you must first determine if you are fertile enough and increase the chances of fertility. One way to do that would be to practice fertility yoga. Another is to monitor your ovulation cycle. Within that time, you should look for signs of high fertility in a woman. How long after ovulation can you take a pregnancy test? Two weeks is the recommended duration.

Get Double the Info with SneakPeek 

You might boost your chances of having twins if you’re undergoing fertility treatments, eat a diet high in animal products, conceive while breastfeeding, or continue to grow your family tree.

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Once you conceive, call on SneakPeek, and get the first introduction to your bundle (or bundles) of joy.