A New Jersey teenager delivers a baby in a school bathroom and then joins her boyfriend on the dance floor of her high school prom. Another teen delivers a baby in the garage of her parents’ house.

As this week’s events add to recent reports of adolescents secretly carrying pregnancies to term, families wonder how a teen can hide a pregnancy for nine months. How can an event that causes most women great physical changes go undetected?

“Sometimes with a first pregnancy you have very strong stomach musculature,” said Susan Tew, a researcher at the Alan Guttmacher Institute in New York, a nonprofit research organization that studies issues related to reproductive health. “The showing is not as extensive as someone who is older.”

And teens, she said, “wear baggy clothes and avoid situations where they may be seen dressing.”

“You have a lot of teens spending a lot of time in their bedrooms and hiding from everyone else - that may be seen as just normal behavior at that time,” she said.

Secret pregnancies, doctors say, simply confirm that much of pregnancy is mental.

“A lot of this has to do with denial and conscious suppression of the consequences of the pregnancy,” said Phillip Goldstein, chairman of obstetrics and gynecology at the Washington Hospital Center in Washington. “Granted there is no way to ignore kicking in your abdomen from the inside out, but denial and ignorance and guilt are part of it.”

Doctors say such births are not uncommon and can involve women of any age, not just teenagers. So can the opposite. Goldstein said he has seen women come into the hospital believing they are in labor when they were not even pregnant.

But whether the teens in the recent cases were in denial or not, how did their families fail to notice the pregnancy?

Goldstein, who is also on the faculty of Johns Hopkins University, advised parents to keep lines of communication open with their teens, and not rely on their ability to spot a pregnancy: “Even though everyone says ‘I can tell when someone is pregnant,’ they really can’t.”