Here’s How Some Women Get Pregnant When They’re Already Pregnant

There are three things that happen to your body when you get pregnant that make it super unlikely you’ll be able to get pregnant again in the next 9 months:

1. You stop ovulating. You need to produce a healthy egg in order to get pregnant. Once that egg has been successfully fertilized and implants in your uterus, pregnancy hormones tell your ovaries that you don’t need to ovulate anymore right now.
2. Speaking of your uterus, it gets pretty tough for another fertilized egg to implant once the first one has nestled in there. The uterine lining thickens to support the first egg and that makes it hard for another to attach itself.
3. During pregnancy, your cervix produces something called a mucus plug, which not only protects your uterus from infection but also prevents sperm from passing through the cervix.

Any one of these things — ovulation, second implantation, or sperm getting through in the first place — happening after conception would be unusual.

Having all three of them happen, resulting in superfetation, is practically unheard of. (We mean this literally: Medical experts can only point to about 10 confirmed cases in the literature, as evidenced by a 2017 article.)

In order to have a double pregnancy, you’d have to either ovulate while pregnant or have two uteri. Both of those scenarios, again, are highly unlikely.

Ovulating during pregnancy happens so infrequently doctors haven’t been able to study why it may occur.

While uterine abnormalities aren’t quite as uncommon, doctors typically see people with a divided or partially formed uterus, not two separate uteri.

This condition, called didelphic uterus, is rare. While it can cause a double pregnancy, it’s more likely to result in a miscarriage than two pregnancies at the same time.

Since double pregnancy happens so infrequently, there’s no definitive info about how close in gestational age the two fetuses would be.

A 2013 study suggests these fetuses are usually conceived between 2 and 4 weeks apart, so it’s likely to be something that happens within a short window of time after the first conception. Considering that the average length of time between menstrual cycles is about 28 days, that makes sense.

As far as the timing of labor and delivery, a double pregnancy can complicate things a little but not dramatically. You wouldn’t be dealing with, say, a 7-month-old fetus and a 3-month-old one.

Your babies will be close in age. For the most part, babies born between 37 and 38 weeks gestation have healthy outcomes, so you could — in theory — schedule a delivery that falls somewhere in between the younger and older babies’ estimated due dates.

There have been a handful of confirmed double pregnancies over the years, including:

• Jessica Allen agreed to be a surrogate for a Chinese couple. When it was discovered she was carrying two fetuses, doctors assumed the embryo had split into twins. After delivering the babies, however, both Allen and the biological parents were confused about how different in appearance they were. DNA testing eventually confirmed that one baby was the biological child of Allen and her husband while the other was the biological child of the Chinese parents.
• Julia Grovenburg became pregnant with one baby in early 2010 and then conceived another roughly 2 and a half weeks later. The superfetation was discovered by her doctor during an ultrasound, which revealed the babies were growing at two different rates within two different uterine sacs. The babies also had two different due dates but were ultimately born via cesarean section on the same day.
• Kate Hill conceived two babies 10 days apart after receiving treatment for polycystic ovary syndrome. She and her husband were trying to conceive but only had sex once — despite the two eggs being fertilized separately.

Twins happen when a fertilized egg splits in two after implantation (for identical twins) or when two separate eggs are fertilized at the same exact time (for fraternal twins).

These are different from superfetation, which occurs when two eggs are fertilized during separate instances of ovulation.

In other words, twins are conceived during the same ovulatory cycle. In superfetation, one egg is fertilized and implants in the uterus, and then — during a secondary ovulatory cycle — another egg follows suit.

As far as knowing when a double pregnancy has occurred instead of the more likely twin conception, it’s fairly tough to decipher before the babies are born.

Two of the indicators — a significant difference in gestational size and a second baby suddenly appearing on a later ultrasound — can have other explanations. For example, it’s more reasonable to assume that the fetuses are simply growing differently or that an ultrasound technician missed the second fetus the first time around.

After birth, of course, a marked difference in the babies’ physical appearance (like being of two different ethnicities, as in Jessica Allen’s case) is a strong enough sign that DNA testing may be warranted, which would confirm or rule out superfetation for certain.

Further complicating things, there’s a similar-but-different biological phenomenon called superfecundation, which refers to fraternal twins with two different fathers.

This happens when two eggs are released during one ovulatory cycle, with each one being fertilized by sperm from a different male partner. A woman would need to have sex with two different men within the short window of ovulation, which is typically about 5 days.

Since the eggs are released, fertilized, and implanted during the same ovulatory cycle, superfecundation isn’t the same as a double pregnancy. It’s almost equally as rare, though. One study from way back in 1992 estimated it had happened in about 2 percent of the twin cases examined.

Once more for the people in the back: This situation happens so infrequently that doctors can’t say if the risks of carrying and delivering babies with a double pregnancy are higher or not than in traditional pregnancies.

If both fetuses are developing normally, there may not be any increased risks in carrying them. On the other hand, problems may arise if one is significantly “younger” in gestational age or less developed than the other.

Beyond that, a person facing delivery with a double pregnancy would simply have the same risks as anyone delivering multiples. Those risks include low birth weight, preeclampsia, and preterm delivery, among others.

Do you need to be worried about winding up with a superfetation situation? Probably not. It can happen once in a blue moon — and if you’re an extremely rare case, it could explain why your “twins” aren’t developing along the same growth pattern.

Otherwise, consider this a fun fact to pull out at parties: Yes, you can (in theory) become pregnant while pregnant.