“Paternal twins” is not a medical term. The phrase people usually mean is “fraternal twins,” sometimes confused because “fraternal” and “paternal” sound similar. Fraternal twins, known medically as dizygotic twins, form when two separate eggs are fertilized by two separate sperm during the same menstrual cycle. They share about 50% of their DNA, the same as any siblings born years apart. They just happen to share a womb.
Why the Term “Paternal” Causes Confusion
The word “fraternal” comes from the Latin word for “brother,” which can make it sound like it has something to do with the father’s side. It doesn’t. Fraternal simply means the twins are not genetically identical. The correct scientific name is dizygotic, meaning “from two zygotes” (two fertilized eggs). Identical twins, by contrast, are monozygotic: one fertilized egg that splits into two embryos.
There is one rare scenario where fathers are directly relevant to twinning. In a phenomenon called heteropaternal superfecundation, a woman releases two eggs in the same cycle and each egg is fertilized by sperm from a different man during separate acts of intercourse. This results in fraternal twins who have different biological fathers. It’s extraordinarily uncommon. As of 2020, only 19 cases had been reported worldwide, and it’s likely underreported because it only comes to light when paternity is disputed.
How Fraternal Twins Form
Normally, a woman’s ovaries release one egg per menstrual cycle. Fraternal twinning happens when the ovaries release two eggs instead, a process called hyperovulation. If both eggs encounter sperm and implant in the uterus, the result is two genetically distinct embryos developing side by side.
Hyperovulation is driven by the hormone that triggers egg development in the ovaries. When levels of this hormone rise higher than usual, or stay elevated for longer, multiple follicles can mature at once and release more than one egg. Mothers of fraternal twins tend to have a natural predisposition toward this pattern of multiple egg release.
What Makes Fraternal Twins More Likely
Several factors increase the odds of conceiving fraternal twins.
Family history. Fraternal twinning runs in families, particularly on the mother’s side. If you have a close relative, like a sister, who had fraternal twins, your own chances roughly double compared to the general population. This makes sense biologically: hyperovulation is the inherited trait, and only the person ovulating can pass it along. A father can carry genes for hyperovulation and pass them to a daughter, but those genes have no effect on his own ability to father twins.
Maternal age. Women in their 30s are more likely to conceive fraternal twins. Research from the University of Western Australia suggests this is an evolutionary response to declining embryo viability with age. As women approach the end of their reproductive years, the majority are likely releasing two eggs per cycle, though only a small fraction of those double ovulations result in twin births. This finding helps explain why fraternal twinning rates have risen as more women delay childbearing.
Fertility treatments. Medications that stimulate the ovaries dramatically increase the chance of multiples. The risk varies by treatment type: single-embryo IVF transfers carry about a 3% chance of multiples, oral ovulation-stimulating medications around 8%, and injectable hormones as high as 30%. The number of eggs recruited and the woman’s age matter more than the specific drug used.
How Fraternal Twins Differ From Identical Twins
Identical twins start as a single fertilized egg that splits into two embryos, so they share virtually all of their DNA. They are always the same sex, and they typically look strikingly similar. Fraternal twins, on the other hand, are genetic siblings who happened to be conceived at the same time. They can be the same sex or different sexes. They may look alike, or they may look no more similar than brothers and sisters born years apart.
In the U.S., the overall twin birth rate is about 31 per 1,000 live births. Fraternal twins account for roughly two-thirds of all twin pairs, making them about twice as common as identical twins.
How Twin Type Is Confirmed
For same-sex twins, it’s not always obvious from appearance alone whether they’re fraternal or identical. DNA testing is the gold standard, with 99.99% accuracy. The process is simple: a swab collects cells from the inside of each twin’s cheek, and a lab compares 12 specific regions of their DNA. Identical twins will match at all 12 regions. Fraternal twins may share up to five marker patterns but will differ on the rest, just like any pair of siblings.
Opposite-sex twins are always fraternal. Since identical twins come from the same fertilized egg, they carry the same sex chromosomes.
One More Rare Possibility: Superfetation
In an even rarer twist than twins with different fathers, superfetation occurs when someone becomes pregnant while already pregnant. A second egg is released, fertilized, and implants weeks after the first embryo. The result is two babies at slightly different stages of development, typically separated by two to four weeks of gestational age. Only about 10 confirmed cases exist in the medical literature. Most have involved fertility treatments, where hormonal stimulation created conditions unusual enough for a second conception to slip through.