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Superfetation: Can you get pregnant while you’re already pregnant?

Superfetation: Can you get pregnant while you’re already pregnant?

As strange as it may sound, it is possible to get pregnant even if you are already pregnant! It seems bizarre, right? Well, as it turns out, a condition called ‘superfetation’ can cause a pregnant woman to get pregnant again while carrying a baby. Ultimately, the woman gives birth to twins who might be from a few days to a few weeks apart.

However, it should be noted that the phenomenon of superfetation is extremely rare and creates a buzz whenever it happens. For instance, when a Brisbane woman became pregnant twice due to superfetation, it created a buzz all over Australia!

Unfortunately, the reasons for the superfetation are still not understood completely. Let’s explore a bit more about this very interesting phenomenon, the processes involved, and the risks associated.

Can You Get Pregnant While Pregnant? It's Rare But Possible
Can You Get Pregnant While Pregnant? It’s Rare But Possible

What is superfetation?

As we saw earlier, superfetation is extremely rare, and as of 2022, there are only about ten documented cases in humans. As superfetation is so rare, not a lot of literature is available on the reasons and causes that lead to such incidents.

Apart from humans, nearly all mammals exhibit superfetation at a significantly higher rate. Superfetation is common in Hares, panthers, and even buffalos. As human reproductive biology differs from other mammals in some key aspects, superfetation is such a rare phenomenon.

In case of a successful superfetation, the pregnant woman gives birth to two babies, but one can be developmentally ahead by several weeks. Hence, the babies are born at different times.

What is the likelihood of you experiencing superfetation?

Unfortunately, there is not enough data to predict the frequency of getting pregnant twice. However, it would be safe to assume that the odds of you having twins due to superfetation are extremely small. According to some sources, the chance is one in several million.

The science behind getting pregnant twice

One of the most important prerequisites for superfetation is ovulation. Normally, your menstrual cycle stops temporarily when you are pregnant, and there is no ovulation until you give birth. However, your ovaries must release an egg while you are already pregnant to get pregnant twice. Hormones including human chorionic gonadotropin, estrogen, and progesterone would prevent second ovulation once the fetus is implanted in the uterus, preventing superfetation.

Even if the second egg is released, it still needs to be fertilized by the sperm for you to get pregnant. The sperm has to move through the vagina and cervix, and uterus into the fallopian tube to make it happen.

Once you are pregnant, the consistency and texture of the cervical mucus change, and it becomes thick and sticky, making it quite difficult for the sperm to swim up to the fallopian tubes. The mucus forms a ‘mucous plug’ that prevents bacteria and other infections from reaching the fetus. For superfetation to occur, the second sperm must travel through this barrier to reach and fertilize the egg, the possibility of which is extremely slight.

Another barrier to superfetation exists in the form of a space crunch in the already occupied uterus. One of the fetuses is already implanted, so finding a suitable space for the second implantation can be a huge challenge even if fertilization occurs. The situation gets even trickier as once a fetus is already implanted, the body stops producing key hormones that aid in subsequent implantations.

As you can imagine, overcoming all these three barriers is extremely difficult, if not impossible. Hence, superfetation is such a rare phenomenon.

How rare is a double pregnancy superfetation?
How rare is a double pregnancy superfetation?

Are there any signs or symptoms of superfetation?

There are no signs or symptoms that can alert an expecting mother about superfetation. You will not notice any unusual symptoms, and it is not until your doctor’s visit that you will know that you are expecting twins due to superfetation.

Even your doctor might not notice that the twins you are carrying are due to superfetation during the early weeks of pregnancy. The doctor might assume that you are having twins. However, as the pregnancy progresses, you can see the difference in the fetuses’ growth rates. One might appear bigger than the other as one was implanted before the other.

The condition where one of the twin fetuses is smaller and shows delayed development than the other is called twin growth discordance. There can be different reasons for twin growth discordance, such as placental insufficiency and intrauterine growth restriction.

Consequently, it might take time to figure out that the twins you are carrying are due to superfetation.

Is there a specific cause of superfetation?

There is no real cause that can be attributed to superfetation. However, in most reported cases, the women experiencing superfetation were on certain fertility treatments, including IVF.

One of the reasons why superfetation can happen after IVF is because it is still possible for a female undergoing the procedure to ovulate even after a fertilized egg is transferred to the uterus. If the second egg is released, the woman can get pregnant again soon after an IVF procedure.

Another probable reason given for superfetation is the initial fetus taking more time than usual to implant itself in the uterine endometrium. If the implantation is delayed, it can postpone the surge of pregnancy hormones and a chance for second ovulation, although very slim.

When does superfetation occur?

If at all happens, superfetation occurs within a few weeks of the first pregnancy. However, due to very limited medical research on the topic, we don’t have any concrete predictions.

Are there risks of superfetation?

Superfetation does carry some inherent risks, even though a few successful cases have been reported recently. As the second fetus is developmentally behind the first one, there is an increased chance of premature birth of the second fetus. Being born prematurely, the baby might have to face certain health issues, including-

  • Breathing problems
  • Digestive problems
  • Delay in the overall development, including neuromotor development

Apart from the risks to the babies, expecting mothers might also face risks, including an increased chance of developing preeclampsia, gestational diabetes, and even iron deficiency leading to anemia.

Can you ovulate when you are pregnant?

In most cases, a pregnant woman can’t ovulate. Once you are pregnant, i.e., implantation occurs, your body increases the production of certain hormones called pregnancy hormones. Progesterone, hCG, and estrogen focus all the body’s resources to support the pregnancy, and the processes that lead to ovulation are put on hold.

However, in the case of superfetation, the body somehow finds a way to circumvent these blocks, and ovulation occurs even when a fetus is successfully implanted.

Can you still ovulate in the early stages of pregnancy
Can you still ovulate in the early stages of pregnancy

Stories of women getting pregnant while pregnant

Kate Hill

The first anecdote comes from Australia itself. Kate and Peter Hill have been trying to conceive for a few years as Kate struggles with the polycystic ovarian syndrome (PCOS). The couple decided to take the help of fertility experts and started their fertility treatment sometime in 2015. Fortunately, Kate got pregnant in 2016.

Two weeks after getting pregnant, Kate got pregnant again due to superfetation. The couple welcomed non-identical twins via c-section, and both the babies were healthy. Kate and Peter recall their journey in this BBC article, full of excitement and anxiety.

Rebeca Roberts

Another case of superfetation comes from the UK, where Rebecca Roberts gave birth to twins in 2020. Rebecca found out about her superfetation twin at her regular 12-week appointment, and the doctors concluded that the twins were approximately three weeks apart.

Rebecca managed to carry the pregnancy to the 33rd week, after which labour had to be induced to prevent any complications. She gave birth to two babies, a son, and a daughter. As the daughter was born in the 30th week, she had to spend around 90 days in the NICU. Thankfully, both the children are in perfect health.

Rebecca’s case was covered extensively worldwide, and three weeks apart, the twins are considered the biggest age difference among reported superfetation cases.