About Fraternal Twins (Dizygotic Twins)

Fraternal twins, those with genes that are different, are the result of double ovulation or the release of two eggs by the mother during ovulation. The two eggs are then fertilized by two sperm. Fraternal twins make up about two-thirds of all twin births.

Fraternal twins may or may not look alike--the same way other siblings born at different times may or may not bear a resemblance to one another. They may share anywhere between 25 to 75 percent of their genes, and may have different blood types. They can be either girls or boys or a combination.

It is believed that having the predisposition to release more than one egg at a time is genetic and, therefore, having fraternal twins can run in families. This occurrence is also more common in women using fertility treatments or those who are over 35-40 years old when they become pregnant.

How do you determine if your twins are fraternal?

The zygosity--whether your twins are identical or fraternal--can be determined before birth through genetic tests like amniocentisis or chorionic villus sampling. If you are interested in finding out before your twins are born, ask your doctor about these tests and the risks or benefits associated with them.

Many parents of twins are told that the presence of two placentas means their twins are fraternal. This isn't true. Some identical twins are also carried with separate placentas. In addition, sometimes the two placentas can become fused together, appearing to be one. Looking at your twins' appearance isn't foolproof either since some fraternal twins look very similar--sometimes more so than some sets of identical twins!

If the zygosity of your twins isn't clear at birth, you can ask your doctor to send a sample of the placenta for testing. You can also have DNA fingerprinting done at a later date to determine your twins' zygosity for a relatively low cost. Ask your doctor about your options.

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Sean and Nicholas Not rated yet
My little sweeties were born five weeks early on May 19th, 2010. My baby Sean was quite a bit smaller than his twin brother Nicholas. Sean was only 3 lbs., …

Autumn and Blair Not rated yet
My girls are 2 year olds. They were born Jan. 3, 2009. They were born 6 weeks early. They stayed in the NICU for 2 weeks. They left NICU at 4.9 and 4.6 …

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