August 20, 2025, 10:20 pm | Read time: 4 minutes
It happens very rarely, but it can occur: Women can become pregnant a second time during pregnancy—sometimes even by two different men. This phenomenon is known as superfetation. But how is it possible, and what does it mean for the mother and child(ren)? A gynecologist has answers.
In 2020, a woman in China had twins. A DNA test delivered a shocking revelation to the presumed father: The children are not siblings but half-siblings! They have two different fathers. A similar case made headlines in the U.S. in 2015. Here, too, a woman had fraternal twins by two different men. She had sex with them a few days apart. This is referred to as superfetation—when a woman becomes pregnant twice in the same cycle.
Superfetation can also occur with the same partner. A woman from the U.S. shared on TikTok her experience with superfetation. She and her fiancé were thrilled to learn about the first pregnancy. Then came the shock: In addition to the first fetus, twins suddenly appeared on the ultrasound.
“It is very rare, but it is possible,” confirms Dr. Mandy Mangler, chief physician of the Department of Gynecology and Obstetrics at Berlin’s Vivantes Auguste-Viktoria Hospital.
Pregnant Again During Pregnancy—How Can That Happen?
Superfetation is distinguished from superfecundation. In superfecundation, an egg is fertilized and, while on its way to the uterus, another egg is released and also fertilized. If you only have sex with one man, you probably won’t notice. “The woman is simply pregnant with twins,” explains Dr. Mandy Mangler.
In superfetation, the pregnancy is already implanted, unlike superfecundation. This means the fertilized egg has already nested in the uterus. If another egg is released and fertilized, it is called superfetation. “This is almost impossible in humans,” says the gynecologist. “Normally, a woman’s hormone levels change so quickly after implantation that no further egg can be released.”
Superfetation Due to Multiple Ovulations
Typically, a woman has a single ovulation during her cycle. However, multiple ovulations are not uncommon. This phenomenon is said to occur in about ten percent of women of childbearing age. A double ovulation usually happens within a time frame of about six to 24 hours—this is how twin pregnancies typically occur. If the second ovulation and its fertilization happen not within a few hours but after several days, it is due to an elevated hormone level of the follicle-stimulating hormone. This prevents pregnancy hormones from being released immediately after the first egg is fertilized. The body is essentially unaware of the pregnancy—and allows the fertilization of another egg.
Recognizing Superfetation
Superfetation is often only noticed when a DNA test reveals that the twins have two different fathers. Sometimes it can be detected during pregnancy. An ultrasound may show that the children are of different sizes. “This can be an indication of superfetation. However, it could also be that the twins are simply developing differently,” the gynecologist cautions. “An ultrasound does not provide a definitive result.”
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Risks of Superfetation
Superfetation is not inherently dangerous. However, it means that one of the twins—the one conceived later—will be born slightly early. “You cannot deliver the children at different times; when one child is born, the other automatically comes into the world as well,” says Dr. Mandy Mangler. “This is due to hormones. They cause both children to be born at the same time.”
Malformations Favor Superfetation
Superfetation occurs so rarely that there is no need to fear it, emphasizes the expert in gynecology and obstetrics. An exception is women with two uteruses or two half uteruses or two vaginas. In these cases, it can indeed happen that two pregnancies occur in two different uteruses.