Shocker! A Texas mother of twins finds out her 11-month-old boys do not have the same father. How could this happen? An OB/GYN fills us in.

Mom Mia Washington noticed that her twins Justin and Jordan had different facial features, so she decided to get it checked out.

A paternity test revealed that two eggs had been fertilized by two different sperm and there was a 99.99% chance the twins had different dads, reports Sky News.

Mama Mia!

It turns out Washington had an affair, and got pregnant by two different men at the same time. "Out of all people in America and of all people in the world, it had to happen to me," she told myfoxdfw.com. "I'm very shocked."

How rare is this? We asked OB/GYN Dr. Suzanne Gilberg-Lenz to shed some light:

Twins with different fathers (also called "bi-paternal twinning") can happen in either assisted pregnancies (when fertility treatments are used) or unassisted pregnancies (when pregnancies happen without the use of fertility drugs or treatments).

Fraternal twins happen when two of mom's eggs are fertilized by two different sperm. Some people assume that fraternal twin fertilization occurs during one single act of sex. This doesn't have to be the case at all. One egg can be fertilized during one "encounter," and the second egg can be fertilized at a later time as a result of a subsequent "encounter." The length of time between fertilizing the eggs can actually involve more than one menstrual cycle. This means that if mom had sex with more than one person during the time she became pregnant with twins, the twins could have two different fathers. Identical twins, on the other hand, are the result of one egg fertilized by one sperm that then splits in half. Identical twins cannot have different fathers.

Twins with different fathers were once thought to be very rare. However, one recent study estimates that, worldwide, one in 12 fraternal twin sets are bi-paternal. (They even have a special name for it: Heteropaternal Superfecundation.) The study suggests cultural standards, surrogacy pregnancy and fertility procedures, Mom's sexual activity, and perhaps even prostitution accounts for this frequency. The study also suggests that among married white women in the U.S., one pair of fraternal twin sets in 400 is bi-paternal. Documented cases of twins with two different fathers go back as far as 1810 in the United States.

What do you think of the twins with two dads? Comment below!

Dr. Suzanne Gilberg-Lenz completed her undergraduate education at Wesleyan University and post-baccalaureate pre-med studies at Mills College. She earned her medical degree from the USC School of Medicine and has been in private practice for 9 years. She is the co-founder of Cedar Sinai Medical Center's Green Committee and lives in Los Angeles with her husband and two children.