A viral post that claims the ballooning size of a uterus during a period is to blame for a “heavy” feeling many women experience isn’t backed by science.

A viral photo making its way around Facebook claims to explain the reason many women feel “heavy” during their periods.

The photo compares two uterus models side-by-side. One represents a smaller, nonmenstruating uterus. The other model is a dark-colored menstruating uterus almost double in size.

The claim comes from Apples and Ovaries, an account run by a holistic nutritional therapist and natural fertility health consultant.

“This is why we feel so heavy at the beginning of our bleed. Why it can feel like our uterus is about to drop out and hit the pavement… And why we need to take things slooooooow,” Apples and Ovaries wrote as a caption for the photo.

Nearly 25,000 people have shared the Facebook post and 19,000 have liked it. The resounding response in more than 6,000 comments is, “Well that explains it.”

One woman wrote, “Ahhh, that is a wonderful confirmation of the discomfort. All makes more sense now.”

Another woman wrote, “WOW good to know cause I do feel so bloated and heavy on the 2nd day.”

But two gynecologists say the models aren’t grounded in science.

Healthline asked Dr. Safrir Neuwirth, an OB-GYN at CentraState Healthcare System, if a woman’s uterus nearly doubles in size monthly during her period.

“The simple answer is no,” Neuwirth responded. “In my 20 years of practice, I’ve never noticed much of a change in the size of the uterus during the period.”

Another OB-GYN, Dr. Kimberly Gecsi, who practices at University Hospitals Cleveland Medical Center, had a similar reaction. “I’ve never heard anything about a uterus increasing in size during a woman’s period,” she said.

“There are certain things that happen physiologically during a woman’s period that do increase the volume of the uterus and make it slightly more swollen,” Neuwirth noted. “But does the uterus double in size like this picture? No way.”

Neuwirth explained that there’s increased blood flow to the uterus during that time of the month, driven by a surge in hormones.

The lining of the uterus, which is what sheds during a period, also thickens by about half a centimeter leading up to the first day of menstruation.

“The combination of the two factors might slightly increase the volume of the uterus by as much as 10 to 15 percent,” Neuwirth told Healthline.

Megan Assaf, a licensed massage therapist and the founder of Wombs for Wisdom, created these uterus models. She said the two sizes are “based on a unique combination of medical science data, medical illustrations, cadaver observation, over a decade of manual palpation of living wombs, and concepts from traditional folk healing.”

Assaf told Healthline that the medical basis she cited comes from a Maya Abdominal Therapy book written by Rosita Arvigo, DN.

According to Assaf, the book states that in a woman who isn’t pregnant, the uterus weighs 4 ounces. During menstruation, the uterus can weigh as much as 8 ounces.

However, changes in weight and size aren’t the same thing.

The typical size of a uterus is approximately 7 centimeters long, 5 centimeters wide, and 4 centimeters thick.

But the size varies considerably, emphasized Neuwirth.

“The uterus models in the picture are both totally normal uterine sizes,” Gecsi clarified. “The smaller one looks like a uterus you could see in someone who’s never had children or is postmenopausal, and the other is pretty representative of a uterus in a woman who’s had a few children.”

Neuwirth further explained that having babies increases the size of the uterus, or womb. Most of the time after childbirth, the uterus will remain larger, he said.

There are a few other reasons for the variety in uterus size. About 20 to 80 percent of women will develop uterine fibroids by the time they’re 50. Fibroids are noncancerous growths on the side of the uterus wall. They vary in size and can enlarge the uterus overall.

Genetics are also at play in uterus size.

“It’s important know that if a woman’s uterus is enlarged, it could be a sign of a different problem” and to consult a doctor, Gecsi warned.

She pointed out that since the uterus is set deep into the pelvis, a woman wouldn’t be able to notice the change in size herself.

It’s easy for things to go viral without being fact checked, and these models certainly look compelling.

But the lack of scientific evidence to back up the change in uterine size during menstruation highlights the need to research medical information you read on social media if it’s not from a trusted source.

Since feeling “heavy” during a period is common, the picture of the uterus models resonated with thousands of women.

While women might experience a heavier feeling because of the increased volume of blood in the uterus, Neuwirth said, the main factor behind that full, uncomfortable feeling comes from a combination of bloating, extra water retention, and excess gas.

You can blame those symptoms on the plummeting levels of the hormone progesterone during a period — not a ballooning uterus.