A woman in England gave birth to rare twins born three weeks apart. That means she got pregnant while she was already pregnant.

When a woman becomes pregnant, her body sends several biological processes to prevent a second pregnancy – including the release of hormones that inhibit ovulation. 

Live Science said a pregnant woman could continue to ovulate or release an egg in rare cases, which can then be fertilized by sperm and inserted in the uterus. "Superfetation" is a rare condition in which two fertilized eggs are inserted into the uterus at different times.

But in this new case, Good Morning America said Roberts gave birth to the twins three weeks apart. The woman, 39, got pregnant for the first time last year after trying to conceive, taking fertility medicines for a long time.

During an ultrasound at 12 weeks of pregnancy, doctors found a second baby with a three-week size difference from the first. Roberts' doctors couldn't understand the size discrepancy between the two babies at first because superfetation is rare.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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"My initial reaction was how had I missed the second twin," Dr. David Walker, an OB-GYN at Royal United Hospital in Bath, told Good Morning America. "And following this [I] was slightly relieved that it was not my mistake but a quite extraordinary pregnancy."

Roberts was diagnosed with superfetation, and the doctors warned her that the younger child could not survive. The doctors induced the labor when Roberts was 33 weeks pregnant last September because the younger twin, Rosalie, had stopped developing properly due to a problem with the umbilical cord.

Noah, the older twin, spent three weeks in the neonatal intensive care unit (NICU), while Rosalie remained for 95 days. Both babies are now well and at home.

"When we lay them down next to each other, it's like they instantly know — and they reach out and touch each other's faces, and it's just the most beautiful thing," Roberts told GMA.

She said that twins generally have an incredible bond. But the story between these two, she added, would make them feel much more unique when they are old enough to understand.

The Live Science report informed that superfetation cases can go undetected because the fetuses are so similar in age. Therefore, they're mistaken for ordinary twins. Healthline said most known superfetation instances include patients who used assisted reproductive techniques such as in-vitro fertilization.

roberts-3-ht-er-210330_1617130068187_hpEmbed_4x3_992 Rebecca Roberts and Rhys Weaver pose with their twin newborns, Noah and Rosalie. Rebecca Roberts/Instagram