Pregnant while already pregnant: What is superfetation and how does it happen?

Did you know you can get pregnant with another child while you're already carrying one?

That's what happened to Bettinna Carlos, who recently revealed that her twin babies were conceived separately because of an extremely rare pregnancy phenomenon called superfetation. "Our twins were not from the same egg. In fact, they were not fertilized at the same time (hence the difference in size). Google calls it 'Superfetation twins.' We call it a miracle," she wrote on Instagram. "Nauna 'yung isa, sumunod 'yung pangalawa pero sabay silang lalabas."

According to Cleveland Clinic, the condition is "so rare in humans that there are only about 10 confirmed cases" in the world, making one's chances of being affected by it "close to zero."

How does superfetation happen?

"In superfetation, a new pregnancy begins while another pregnancy is already underway," obstetrician-gynecologist Dr. Sherry Tabije told PhilSTAR L!fe. "It happens when ovulation and fertilization occur several weeks after the first conception, before the walls of the uterine cavity have fully fused or before the uterine cavity is sealed off."

Tabije said you can't consider it as a twin pregnancy, although they are the same in some ways—like how the fetuses are in the same womb, how they develop beside each other, and how they are delivered at the same time.

"But unlike twin pregnancies, embryos resulting from superfetation are not conceived during the same menstrual cycle," she explained. "Because they are conceived at different times, they develop at different stages and so they have different gestational ages."

According to the OB-GYN, the causes of superfetation can be connected to fertility treatments like IVF and ovarian hyperstimulation. "Some are from hormonal irregularities where there’s a failure in the hormonal regulation that prevents the ovaries from releasing new eggs during pregnancy," she added. The doctor, however, noted that further research is needed to confirm its causes.

Is it dangerous?

Per Tabije, diagnosing superfetation is complex.

"It typically involves clinical evaluation, including the patient's history, physical examination, and imaging studies like ultrasound to assess fetal development. A specialist will look for size discrepancy (differences in size and weight) and development mismatch (pointing to two distinct conception dates)," she told L!fe.

Superfetation, while extremely rare, is not inherently dangerous, Tabije noted.

However, the condition may present risks related to asynchronous fetal development such as severe preterm delivery. 

"A planned Cesarean section may increase the chances of a healthy delivery for both babies without complications, especially if one fetus matures more rapidly than the other fetus," she said.

Disclaimer: This article is not meant to replace professional medical advice. Always consult your doctor for any questions you may have.