Giving birth to twins born to different fathers is a rare event. This is what specialists in reproductive biology call “heteropaternal superfecundation”, a phenomenon in which two oocytes, expelled from the ovaries during the same menstrual cycle, are fertilized by sperm from two different men.

This is possible because male gametes can survive for up to five days in the female genital tract. If during this time a woman has sex with two different men and expels two oocytes during the same menstrual cycle, it is possible that one sperm from each of the sexual partners fertilizes an oocyte, resulting in a twin pregnancy. In this case, the dizygotic twins come from two oocytes fertilized by two sperm that do not come from the same progenitor.

While the majority of mammals expel multiple eggs during ovulation, resulting in the delivery of several young, in women, a single egg is usually expelled from one of the two ovaries each cycle. Heteropaternal superfecundation, which involves double ovulation and then successive fertilization during the same menstrual cycle of two oocytes during coitus with two different men, therefore represents a truly exceptional situation.

Known Cases

In 1992, researchers reported that twins with different fathers accounted for one in 13,000 paternity tests. A study published in 1993 also estimated that heteropaternal superfecundation affects one pair of twins in 400 among white women married in the United States.

In 1997, a case of superfecundation was reported by Spanish doctors. In 2015, another superfecundation case was reported in the American legal publication, The New Jersey Law Journal. At that time, only 7 cases of heteropaternal superfecundation were found in the international medical literature. Recently, in Colombia, another case was reported as reported by Scientific Origin.

The first description of this phenomenon was reported in the medical literature in 1810 by John Archer, the first medical doctor to graduate on the American continent in 1798. The situation involved the birth of twins, one of whom was white and the other black, the mother having had relations with two men. Another case was described in 1940 concerning different fathers of the same ethnic origin. Further cases of superfecundation were reported in 1978 in the New England Journal of Medicine and in 1992 in The Lancet.

Superfecundation vs Superfetation 

Heteropaternal superfecundation is not the only intriguing biological phenomenon related to twinning. Twins can also be the result of “superfetation,” a process in which two eggs are fertilized during different menstrual cycles. In other words, superfetation corresponds to the fertilization and implantation of a fertilized oocyte in a uterus already containing the fruit of the fertilization of a previous ovulation. The uterus then contains two fetuses of significantly different ages, but which are ultimately twins.

About ten cases of superfetation have been reported in the medical literature. While superfetation is usual in certain mammals (European hare, European badger, American mink) and freshwater fish, this phenomenon is extremely rare in humans because the development of an ovarian follicle ( and therefore subsequent ovulation) is inhibited as soon as a pregnancy is in progress.

An exceptional case of two concomitant conceptions was reported in the journal Human Reproduction in 2001 by doctors at Stanford University (California) in charge of a 35-year-old woman suffering from infertility for three years. She was immensely surprised to learn that she was going to have quadruplets, even though doctors had only implanted two embryos in her uterus during the in vitro fertilization (IVF) procedure. A spontaneous pregnancy had occurred at the same time, the couple having had sexual intercourse 5 days before the collection of oocytes after ovarian stimulation!

This woman gave birth in the 32nd week of pregnancy to a boy and three girls, all of whom had the same father according to DNA results.

Complications in superfetation

Some cases of superfetation are more exceptional than others, such as the one that occurred in Italy and was reported in 2010 by gynecologists and obstetricians at the University of Turin. It concerns a 32-year-old woman treated for sterility with gonadotropins, drugs that induce ovulation, who underwent artificial insemination with the partner’s sperm. After the third attempt, the procedure was successful, the pregnancy test being positive. But a few days later, the patient was hospitalized with severe pelvic pain. The doctors were surprised to find on vaginal ultrasound the simultaneous presence of early pregnancy (2nd week) and another pregnancy, this one ectopic.

The patient had to remove the fallopian tube where the first embryo had implanted, 4 weeks before the second. The intrauterine pregnancy proceeded normally and ended with the birth of a healthy child in the 39th week.

What had happened? The couple simply had sex that (finally!) led to pregnancy shortly before undergoing a medically assisted procreation (ART) procedure.

Cases of superfetation are usually reported in a twin pregnancy when the ultrasound shows a size mismatch between the two fetuses. Most cases of superfetation now occur after taking drugs to induce ovulation, sometimes combined with assisted reproduction techniques.

Whether it is superfecundation with twins who do not have the same father or superficiality with twins of different gestational ages, the miracle of life indeed hold many surprises.

Erica Delaney

Erica is an experienced nurse working in the central Florida area. She focuses on subjects related to pregnancy and infant health. She is a mother of two with hobbies ranging from dancing to playing the piano.