Superfetation is when a second, new pregnancy occurs during an initial pregnancy. Another ovum (egg) is fertilized by sperm and implanted in the womb days or weeks later than the first one. Babies born from superfetation are often considered twins since they may be born during the same birth on the same day.

Superfetation is common in other animal species, like fish, hares, and badgers. Its likelihood of occurring in humans is controversial. It’s considered extremely rare.

There are only a few cases of supposed superfetation in the medical literature. Most cases occurred in woman undergoing fertility treatments such as in vitro fertilization (IVF).

In humans, a pregnancy occurs when an ovum (egg) is fertilized by sperm. The fertilized ovum then implants itself in a woman’s uterus. For superfetation to happen, another completely different ovum needs to be fertilized and then implanted separately in the womb.

For this to happen successfully, three very unlikely events need to take place:

  1. Ovulation (release of on ovum by an ovary) during an ongoing pregnancy. This is incredibly unlikely because hormones released during pregnancy function to prevent further ovulation.
  2. The second ovum must be fertilized by a sperm cell. This is also unlikely because once a woman is pregnant, their cervix forms a mucus plug that blocks the passage of sperm. This mucus plug is the result of elevations of hormones produced in pregnancy.
  3. The fertilized egg needs to implant in an already pregnant womb. This would be difficult because implantation requires the release of certain hormones that wouldn’t be released if a woman were already pregnant. There is also the issue of having enough space for another embryo.

The chances of these three unlikely events occurring simultaneously seem nearly impossible.

This is why, of the few cases of potential superfetation reported in the medical literature, most have been in women undergoing fertility treatments.

During a fertility treatment, known as in vitro fertilization, fertilized embryos are transferred into a woman’s uterus. Superfetation might happen if the woman also ovulates and the egg becomes fertilized by sperm a few weeks after the embryos are transferred into her uterus.

Because superfetation is so rare, there are no specific symptoms associated with the condition.

Superfetation may be suspected when a doctor notices that twin fetuses are growing at different rates in the womb. During an ultrasound test, a doctor will see that the two fetuses are different sizes. This is called growth discordance.

Still, a doctor probably won’t diagnose a woman with superfetation after seeing that the twins are different in size. This is because there are several more common explanations for growth discordance. One example is when the placenta isn’t able to sufficiently support both fetuses (placental insufficiency). Another explanation is when blood is unevenly distributed between the twins (twin-to-twin transfusion).

The most important complication of superfetation is that the babies will be growing at different stages during the pregnancy. When one baby is ready to be born, the other fetus might not be ready yet. The younger baby would be at risk of being born prematurely.

Premature birth puts the baby at a higher risk of having medical problems, such as:

  • trouble breathing
  • low birth weight
  • movement and coordination problems
  • difficulties with feeding
  • brain hemorrhage, or bleeding in the brain
  • neonatal respiratory distress syndrome, a breathing disorder caused by underdeveloped lungs

In addition, women carrying more than one baby are at increased risk of certain complications, including:

The babies may need to be born via Cesarean section (C-section). The timing of the C-section depends on the difference in the development of the two babies.

You can decrease your chances of superfetation by not having sexual intercourse after you’ve already become pregnant. Still, superfetation is extremely rare. It’s incredibly unlikely that you would become pregnant for a second time if you did have sex after you’ve already become pregnant.

Of the few cases of potential superfetation reported in the medical literature, most have been in women undergoing fertility treatments. You should be tested to make sure you’re not already pregnant before undergoing these treatments, and follow all recommendations from your fertility doctor if undergoing IVF, including certain times of abstinence.

Most reports of superfetation in humans are in women who have undergone fertility treatments to become pregnant.

A case report published in 2005 discusses a 32-year-old woman who had undergone in vitro fertilization and became pregnant with twins. Around five months later, the woman’s doctor noticed during an ultrasound that she was actually pregnant with triplets. The third fetus was much smaller in size. This fetus was found to be three weeks younger than its siblings. The doctors concluded that another fertilization and implantation took place naturally weeks after the in vitro fertilization procedure.

In 2010, there was another case report of a woman with superfetation. The woman was undergoing an artificial insemination (IUI) procedure and was taking medications to stimulate ovulation. It was later found out that she was already pregnant with an ectopic (tubal) pregnancy. Doctors didn’t know the woman was already pregnant with an ectopic pregnancy when they performed the IUI procedure.

In 1999, there was a report of a woman who is believed to have experienced superfetation spontaneously. The fetuses were found to be four weeks apart. The woman went through a normal pregnancy and both babies were born healthy. Twin one was a female born at 39 weeks and twin two was a male born at 35 weeks.

Superfetation is often observed in other animals. The possibility of it happening naturally in a human remains controversial. There have been a few case reports of superfetation in women. Most had been undergoing assisted reproduction techniques, like in vitro fertilization.

Superfetation results in two fetuses with different ages and sizes. Despite this, it’s possible for both babies to be born fully developed and completely healthy.