'Due dates are just a suggestion': Ashley Graham shares full-term pregnancy photo, but is it safe to carry twins to 40 weeks? Experts weigh in

"Due dates are just a suggestion, babies will always come on their birthday."

Pregnant supermodel Ashley Graham kicked off the new year by sharing a photo on her Instagram account with a caption celebrating an almost unheard-of milestone. Graham, who shares an almost 2-year-old son with husband Justin Ervin, has reached her 40-week due date while pregnant with twin boys.

Almost immediately after posting the photos, Graham's comment section was filled with a combination of love and support and calls of worry and distress. Many fans questioned whether carrying to 40 weeks is dangerous for mothers who are expecting multiples, and not without reason: According to Johns Hopkins Medicine, less than half of twin pregnancies reach 37 weeks.

Dr. Christine Greves, a board-certified OB/GYN at the Winnie Palmer Hospital for Women and Babies, says it's certainly an anomaly for women who are pregnant with multiples to remain pregnant up until their due date.

"I've been doing this for almost 20 years and I've seen less than a handful of women pregnant with twins make it to 40 weeks," Greves shares, explaining the reasons twins may be delivered earlier are multifold.

For starters, there's not as much room for two babies inside a woman's uterus as there is for one.

"The uterus is accustomed to getting to a '40-weeks' size and with twins, it typically should measure much farther along much earlier because there's one uterus for two babies," Greves tells Yahoo Life. "Many women will give birth [earlier than 40 weeks] because that's when the uterus is ready and that's when it begins to contract."

Other complications like preeclampsia, diabetes, cord entanglement and selective growth restriction, a condition in which one twin does not receive enough nourishment from the placenta to grow at a normal rate, are just the start of a list of reasons why women and their doctors rarely see twin pregnancies go to full term. But that doesn't mean a 40-week multiple-birth pregnancy like Graham's is unsafe.

"If there's regular prenatal care, she's under the care of her doctors, she doesn't have any risk factors and the babies are looking great during screenings — if her doctors say it is safe — then I would trust that," says Greves.

Greves adds that with any pregnancy, doctors typically do not recommend going past 41 weeks because from there the morbidity rate increases for both mother and child. The Florida-based OB/GYN explains that, according to the Society of Maternal Fetal Medicine and the American College of Obstetricians and Gynecologists (ACOG), the optimal time to give birth is around 38 weeks to avoid major complications.

"Although the data recommends delivery at 38 weeks, that's a recommendation," Greves emphasizes. "And what the patient decides they are comfortable with regarding their risk level for themself and their baby — that's up to them and their doctor."

While rare, there are mothers who carry healthy twins to 40 weeks and beyond. Viktorya Howard is a mom of three who carried her twin daughters, Sierrah and Savannah, now 22, to 42 weeks.

"I had no health issues during my pregnancy," says the Kettering, Ohio mom. "Savannah was about 4 pounds, 12 ounces at birth, and Sierrah was exactly 5 pounds."

"I had a midwife who was amazing," Howard continues. "She never pressured me to deliver earlier: She added weekly stress tests to make sure the girls were both doing well but she always assured me that the girls would come when they were ready."

Greves says, like any other pregnancy, the only people truly able to judge the health of a multiple birth pregnancy are parents and the medical provider they've chosen to provide prenatal care.

"It's incredibly rare that a woman with twins or multiple gestations has the gift of making it to her due date," Greaves explains. "Because twins have a higher risk of causing complications, it may result in an earlier delivery."

Still, Greves says it's unlikely Graham's fans have anything to worry about.

"[In Graham's case]," Greves adds, "I would feel comfortable assuming there is proper medical care."