By Zaira Salvador BSc, MSc (embryologist).
Last Update: 07/13/2018

Superfetation is defined as a type of twin pregnancy in which conception of a new embryo occurs when the woman is pregnant already. Some refer to it as double pregnancy. The result are fraternal twins of different gestation age.

The odds of superfetation in humans are extremely low. The cases that have occurred in our days are due to the medications administered in fertility treatments to induce ovulation.

Definition

According to the Oxford English Dictionary (OED), the noun superfetation is defined as the occurrence of a second conception during pregnancy, which gives rise to embryos of different ages in the uterus.

Simply put, it is a phenomenon whereby it is possible for a woman to get pregnant while already pregnant. The odds for superfetation to happen in humans are extremely low, whilst it is more common amongst other species like rodents, horses, or sheep.

Superfetation is not the same as superfecundation. The latter refers to the fertilization of two eggs that have been released by the ovary in a single menstrual cycle, resulting in a multiple pregnancy with fraternal twins.

Causes

The main cause of superfetation in humans is the occurrence of a second ovulation during an ongoing pregnancy due to failure of the hormones released during pregnancy to prevent ovulation.

When a woman gets pregnant, high progesterone levels cause a blockage in the ovaries that prevents the follicles to grow. So, errors during this process of hormonal regulation are responsible for superfetation.

Moreover, other two very unlikely events need to take place for a new pregnancy to occur:

  • The second egg cell must be fertilized by a sperm. This is very rare because the cervix forms a mucus plug during pregnancy that blocks the passage of sperm.
  • The lining of the uterus must be receptive. This is very rare because it means that your body is still producing the hormones that cause the endometrial lining to thicken for implantation, and that there is space for a new embryo to attach without affecting the first embryo.

All this unusual phenomena are extremely uncommon and have been associated with fertility treatments as a side effect of the high doses of fertility drugs that IVF patients must administer to stimulate their ovaries.

If you need to undergo IVF to become a mother, we recommend that you generate your Fertility Report now. In 3 simple steps, it will show you a list of clinics that fit your preferences and meet our strict quality criteria. Moreover, you will receive a report via email with useful tips to visit a fertility clinic for the first time.

The fetuses will have different gestational ages and due dates, normally a discordance of 2-4 weeks. In fact, if the woman had sexual intercourse with another man during pregnancy and superfetation occurred, each fetus would have different genetic fathers.

Symptoms, diagnosis & risks

Because superfetation is a very rare event in humans, there are no specific symptoms associated with it. It may be suspected when a doctor notices that twin fetuses are developing at a different growth rate in the womb. This is known as growth discordance.

Growth discordance occurs when two fetuses are different sizes. It can only be detected by means of an ultrasound test. It doesn’t always translate into superfetation. Common causes include placental insufficiency and twin-to-twin transfusion.

In case growth discordance is due to superfetation, it will be diagnosed at 12 weeks pregnant, after the first trimester ultrasound.

In addition to having different sizes, there exist other parameters that may lead to a diagnosis of superfetation: the formation of the yolk sac, fetal heartbeats, and other fetal structures are not synchronized.

If superfetation occurs, your OB/GYN must keep in mind that both babies should be born simultaneously. In most cases, it is medically necessary to induce labor or to deliver them by C-section.

This means that one of them will be born prematurely. For this reason, the due date will be scheduled trying to reduce the number of risks for the “younger” baby as much as possible.

Superfetation cases

The first ever case of superfetation in humans dates back to 1932. After this first record, it is estimated that about 10 more cases took place throughout the 20th Century.

However, the Royal College of Obstetricians and Gynecologists (RCOG) has estimated that, nowadays, an average of 1-2 cases of superfetation occur per year.

The latest cases of superfetation have allowed medical experts to establish a solid association between this phenomenon and Assisted Reproductive Technology (ART), especially when IVF is the technique of choice. Also, when hormone replace therapies are used due to alterations in the function of the pituitary gland.

The most recent case of superfetation took place in 2016. It became specially popular because it was a surrogate pregnancy. The surrogate mother, Jessica Allen, gave birth to two babies after being a surrogate for a Chinese infertile couple. However, only one of the babies was the biological child of the intended parents. The other baby had been conceived naturally by Jessica and her husband.

Recommended reading: What Is Surrogacy & How Does It Work? – Everything You Should Know.

FAQs from users

Can you get pregnant in your third trimester?

By Zaira Salvador BSc, MSc (embryologist).

It is extremely unlikely. If superfetation takes place, it does a few weeks apart due to alterations in the regulation of hormones at the beginning of pregnancy. Moreover, the second pregnancy would be unsustainable after delivery of the first child.

Is superfetation common?

By Zaira Salvador BSc, MSc (embryologist).

Actually, the existence of this phenomenon in humans is deemed unlikely. In is more common, though, in animals, including mice, rats, rabbits, horses, marsupials, cats, and primates.

Why can’t you get pregnant while pregnant?

By Zaira Salvador BSc, MSc (embryologist).

When a woman gets pregnant, her body starts releasing hormones to sustain the pregnancy and to prevent a new pregnancy from happening. Also, the cervix creates a mucus plug that prevents the passage of sperm. That’s why women can’t get pregnant while they are pregnant already, except for the rare event of superfetation.

Suggested for you

As explained above, getting pregnant with fraternal twins has nothing to do with an identical twin pregnancy. The causes and particularities of each type of twin pregnancy are different. Get more info by clicking the following link: Fraternal Twins vs. Identical Twins, What’s the Difference?

Ovulation takes place at a particular stage of the menstrual cycle and is controlled by hormones. If you found this interesting, why don’t check out the following post? Click here: Phases of the Menstrual Cycle.

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Authors and contributors

 Zaira Salvador
BSc, MSc
Embryologist
Bachelor's Degree in Biotechnology from the Polytechnic University of Valencia (UPV). Embryologist specializing in Assisted Procreation, with a Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia (UV) and the Valencian Infertility Institute (IVI). More information
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