Hilary Swank is expecting twins at 48. Doctors say a healthy pregnancy at her age is possible, but can be difficult.

• Hilary Swank announced yesterday that she's pregnant with twins.
• The star is 48 — which health experts consider to be advanced maternal age.

Hilary Swank is pregnant at 48, not just with one baby, but with two.

"This is something that I've been wanting for a long time," the actor said on Wednesday in an appearance on "Good Morning America." "My next thing is, I'm going to be a mom."

Being an older pregnant person and carrying twins both increase the risk for pregnancy complications, Dr. Kecia Gaither, who is double board-certified in OBGYN and maternal-fetal medicine and is the director of perinatal services and maternal-fetal medicine at NYC Health + Hospitals/Lincoln in the Bronx, said.

Older moms are at increased risk for complications including gestational diabetes, high blood pressure, and preeclampsia. They're also more likely to miscarry, experience stillbirth, or undergo preterm labor.

However, it's possible for an older mom like Swank to safely deliver twins, Dr. G. Thomas Ruiz, the OBGYN lead at MemorialCare Orange Coast Medical Center in Fountain Valley, California, said.

"As long as the patient understands the complications and risks prior to pregnancy and is willing to go along with everything, you can have a healthy baby and mom at 48 years," Ruiz said. "It's just more work for the mom."

Pregnancies in older people need more monitoring

Swank is likely working with a high-risk perinatologist, according to Ruiz. That's because it's standard for older moms and people carrying multiples to have more frequent appointments, including ultrasounds every three to four weeks.

"We're doing all the surveillance necessary so we can stay on top of any complications," Ruiz said.

Doctors closely monitor a person's fluid levels, cervix length, and other measures of a healthy pregnancy. It's also not uncommon for moms to be put on modified bed rest in the third trimester to prevent preterm labor, Ruiz said.

A spontaneous pregnancy at 48 is unlikely

The chances of a 48-year-old getting pregnant without intervention is about 5%, Gaither said. The likelihood of a healthy pregnancy is much less, Ruiz said, since genetic abnormalities in the fetus increase as the mother ages. For example, by the time a pregnant person is 45, their chances of conceiving a fetus with Down syndrome are one in three.

Because genetic abnormalities in the fetus increase the risk of miscarriage, older moms are more likely to miscarry. Interestingly, the odds of naturally conceiving twins increase with age.

Some ways around a few of these complications are freezing eggs or using donor eggs.

People considering older pregnancy should understand the risks

Ruiz has personally delivered twins to a 51-year-old mother. While it's becoming more common for older people to try to get pregnant, Ruiz likes to make sure his patients understand both the odds and the risks. If someone over 45 expresses a desire to carry a pregnancy, he sends them for bloodwork and other testing to understand their specific likelihood of getting pregnant.

Then, he's upfront about the physical and emotional toll of pregnancy at an advanced age, including all the monitoring and potential for bed rest.

"We can get you through this safely, with appropriate management and care, as long as you understand it's going to be a harder pregnancy than when you were 28," he said. "You can get through it safely."