Superfetation: Exploring the Science Behind Double Pregnancy

In superfetation, new pregnancy bypasses the body’s natural barriers. These natural barriers are so strong that the chance of occurrence of superfetation is nearly zero
Superfetation is a rare phenomenon that involves a second pregnancy while you are already pregnant. (Unsplash)
Superfetation is a rare phenomenon that involves a second pregnancy while you are already pregnant. (Unsplash)


Superfetation is a rare phenomenon that involves a second pregnancy while you are already pregnant. Rarely, do cases of superfetation often create headlines. Once an embryo is developed inside your uterus, your body prevents subsequent pregnancies. Hence, the chance of experiencing this phenomenon is zero.

The body’s natural barriers prevent new pregnancies while you are already pregnant. In superfetation, new pregnancy bypasses the body’s natural barriers.  These natural barriers are so strong that the chance of occurrence of superfetation is nearly zero. Most of the known cases of superfetation involved assistive reproductive technology.

Are superfetation similar to twin pregnancies?

Superfetation pregnancies share similarities with twin pregnancies in certain aspects. Like twins, both fetuses are developed inside the same uterus and are delivered at the same time. Embryos from superfetation are not formed during the same menstrual cycle. They are in different stages of pregnancy. The embryo that was conceived first will mature compared to the embryo that was conceived later.

Superfetation mostly occurs in other species such as rodents, mammals, and fish. There are only about 10 confirmed cases of superfetation.

What happens in superfetation?

During the first cycle, the ovum is released from your body. When you have sexual intercourse, the sperm fuses with the ovum and forms an embryo. The embryo attaches to the uterine lining and begins to develop. When the same events occur during the next cycle, the new embryo develops and joins the first embryo. This is rare as the body’s natural barriers prevent the second pregnancy once you are already pregnant.

Why someone can’t get pregnant while they are already pregnant?

When you are pregnant, hormones change to prevent the release of the ovum from the ovaries. Ovulation during pregnancy is rare. A mucus plug is formed in the cervix that prevents the entry of the sperm into the uterus. The cervix is the opening between your vagina and uterus. If you have sexual intercourse during pregnancy, sperm would have difficulty reaching the uterus and fallopian tubes where fertilization takes place.

(Representational Image: Wikimedia Commons)
(Representational Image: Wikimedia Commons)

Case studies:

A case report published in 2005 discussed a 32-year-old woman who had become pregnant with twins through in vitro fertilization. Five months later, the doctor noticed that the woman was pregnant with triplets during an ultrasound. The third fetus was smaller and three weeks younger than its siblings. The doctor indicated that fertilization and implantation occurred again weeks following the initial in vitro fertilization procedure.

In 2010, there was another case report. The woman was undergoing an artificial insemination procedure and was taking medications to induce ovulation. Later, the doctors found that she was having ectopic pregnancy. Doctors were not aware that she was already pregnant when they performed this procedure.

In 1999, there was a report of a woman who experienced superfetation through a normal pregnancy. The fetuses were four weeks apart.

Both babies were born healthy. One twin was a female born at 39 weeks and the other twin was a male born at 35 weeks.


Superfetation is difficult to diagnose as it resembles similar conditions involving twins. Two or more fetuses are observed during a routine imaging procedure.

Superfetation can also be a misdiagnosis of other conditions like:

  • Twin-to-twin transfusion syndrome: Twins may unequally share nutrients from their mother. The twin that gets more nutrients grows bigger than the other on imaging.

  • Placental insufficiency: Placenta supplies nutrients and oxygen to the developing fetuses. Due to placental insufficiency, the organ is incapable of delivering nutrients to both the fetuses. As a result, there will be a difference in growth rate among the twin babies.

Ectopic pregnancy (Representational Image: Wikimedia Commons)
Ectopic pregnancy (Representational Image: Wikimedia Commons)
Definition: “Superfetation is defined by the fertilization and the implantation of a second oocyte in a uterus already containing the product of a previous conception”. In other words, getting pregnant a second time while one is already pregnant. Superfetation is an extremely rare condition in humans.
Dr. Munish Raizada, Neonatologist, CEO, MedBound Times


A planned C-section can be done to enhance the chances of healthy delivery. Chances of superfetation can be reduced by not having sexual intercourse once you have become pregnant.

C-section (Representational Image: Wikimedia Commons)
C-section (Representational Image: Wikimedia Commons)


Both babies will be growing at different stages during the pregnancy. When one baby is ready to be born, another baby might not be ready. The younger baby might be born prematurely. Premature birth can lead to:

  • Breathing difficulties

  • Low birth weight

  • Difficulties in feeding

  • Brain hemorrhage

  • Neonatal respiratory distress syndrome

  • Movement and coordination difficulties

Women who are pregnant with more than one baby are at a heightened risk of:

  • Preeclampsia

  • Gestational diabetes


While superfetation is uncommon, it provides valuable insights into the intricate workings of reproductive biology and underscores the remarkable adaptability of the human body. Enhanced comprehension of this phenomenon through continued research could potentially advance fertility treatments and improve reproductive health outcomes. Nonetheless, superfetation's rarity means that it primarily serves as a subject of intrigue rather than a major focus in obstetrics and gynecology.


1.    Fletcher, Dan (28 September 2009). "How Can a Pregnant Woman Get Pregnant Again?". Time Magazine. Retrieved 29 March 2018.

2.    Vince, Gaia (9 July 2003). "Women can ovulate more than once a month". New Scientist. Retrieved 22 October 2018.

3.    Pape, O.; Winer, N.; Paumier, A.; Philippe, H.J.; Flatrès, B.; Boog, G. (4 June 2008). "Superfœtation: about a case and review of the literature". EM Consulte Journal of Obstetrics and Reproductive Biology.

4.     "Glossary - superfetation". FishBase. Archived from the original on 6 December 2017. Retrieved 16 December 2018.


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