03/02/2017 07:11 EST | Updated 03/02/2017 07:18 EST

Women Getting Pregnant While Pregnant Is Extremely Rare

It's a lot more common for animals.

For any pregnant woman who is worried about getting pregnant again while expecting, you should know the chances are extremely rare.

So rare, the The New York Times reports, that only a few human cases have been documented.

Called superfetation, this phenomenon occurs when a woman continues to ovulate after becoming pregnant, Time notes.

woman pregnant

"[The] fertilized egg is able to implant itself in the lining of the womb — two things that wouldn't happen in a normal pregnancy. Typically, hormonal changes prevent further ovulation and thicken the lining of the uterus to preclude a second embryo from attaching," the magazine adds.

Each fetus can also have a separate amniotic sac and be born within weeks apart.

woman ultrasound

In 2009, a 31-year-old Arkansas woman named Julia Grovenburg experienced superfetation firsthand and delivered two healthy babies born two-and-a-half weeks apart, ABC news reports.

"When [Grovenburg] had her ultrasound initially, they saw one sack, one baby developing, and that baby had a certain gestational age; then they noticed a second heartbeat in a child that was much, much younger developmentally," Dr. Karen Boyle of the Greater Baltimore Medical Center told ABC News.

"There is no prevalence or incidence in the literature. I could only find about 10 reported cases."

kate hill

Kate Hill.

And last year, an Australian woman gave birth to twins 10 days apart. Kate Hill had unprotected sex with her husband Peter once, but still managed to conceive twins at two different times.

“We actually did not realize how special [superfetation] was until they were born,” Hill told Australia’s “Today Tonight.”

“What makes this case even more rare, is that my husband and I only had intercourse one time,” she added. “His sperm stayed alive for 10 days to fertilize the second egg released.”