The actress has checked into a French Riviera hospital in Nice, France, to be kept under doctors' watch and rest before she gives birth. Doctors say that's not a bad strategy.

“The risk of nearly everything you can think of in pregnancy is increased with twins,” said Dr. Virginia Beckett, an obstetrician-gynecologist and spokeswoman for Britain's Royal College of Obstetricians and Gynecologists.

Mothers pregnant with twins have higher chances of miscarriage, stillbirth, high blood pressure, and diabetes. Beckett characterized women expecting twins as “a particularly worrying group of patients.”

Twins are also more likely than single-born babies to have chromosomal or genetic abnormalities, since early diagnosis of these problems is difficult.

While there is little that can be done to change your genetics, women pregnant with twins cope with the physical strain on their bodies better if they are healthy and fit.

“I assume Angelina doesn't have a problem with that,” Beckett said.

More than half of all twins are born early, which may explain Jolie's recent check-in at the Lenval Hospital in southern France. Jolie has previously said the twins were due in August.

Doctors aren't entirely sure whether a natural delivery or a Caesarean section is better for twins; an international twin birth study is still ongoing to investigate that question.

In a natural delivery, monitoring the twins externally is hard. Labor is generally straightforward until the second twin arrives.

Once the first twin is out, there is more room for the second twin to maneuver. That extra room may be problematic since it could allow the second twin to get into difficult positions, including a breech birth.

Doctors might recommend a Caesarean section if a woman has high blood pressure, if there is a risk of delivering early, or if previous deliveries have had complications.

Jolie's arrival at the hospital ahead of her delivery date could mean she has some health problems doctors want to monitor.

Her doctors might be monitoring her for pre-eclampsia, experts suggested. The condition causes high blood pressure and an abnormal amount of protein in the urine. It can result in complications including seizures, a stroke, or severe bleeding.

“This is the main one to watch because it's much more common in twin pregnancies,” said Dr. Steven Goldstein, a professor of obstetrics and gynecology at New York University School of Medicine.

Goldstein said pregnant women would not normally be admitted into hospitals before delivery unless there was a medical concern.

“If everything's perfect, then you would not want to deliver early,” Goldstein said. “It's best to carry the twins to term if you can.”

Still, Beckett said Jolie might be perfectly healthy, and said that some women chose to check into medical facilities even with no impending problems.

“Maybe Angelina just needs a break from everything,” she suggested.