The health care overhaul provides a safety net for young adults, who can stay on a their parents’ plans until age 26. But it doesn’t guarantee coverage for a common medical condition: pregnancy.

Group health plans with 15 or more workers must provide maternity benefits for employees and their spouses under the Pregnancy Discrimination Act of 1978. But other dependents of employees aren’t covered by the law.

Hard numbers aren’t available on how many companies don’t offer maternity benefits to dependents. ‘‘I would say it’s common,’’ said Dania­ Palanker, at the National Women’s Law Center. And the number could grow with the recent expansion of coverage to children under age 26.

In 2008, about 2.8 million women ages 15 to 25 got pregnant, 12 percent of all those in this age group, according to the National Center for Health Statistics. (That is the most recent year for which there are pregnancy estimates.)

In some states, a pregnant young woman might qualify for Medicaid, the federal-state program for low-income individuals, even if she lives with her parents.

Under the health care overhaul, preventive benefits that are recommended by the Preventive Services Task Force, a federal agency, must be covered by new plans and by plans that have changed enough to lose their “grandfathered” status. Recommended services include a range of screenings for pregnant women, including for anemia, hepatitis B, and Rh incompatibility. In addition, starting this month, when a non-grandfathered health plan begins its new plan year, it must provide certain other women’s health services at no charge, including an annual well-woman visit, screening for gestational diabetes, and breast-feeding support.

Starting in 2014, maternity and newborn care is one of 10 so-called essential health benefits that must be offered by all health plans in the individual and small-group markets, including those that are sold through the state-based health insurance exchanges that will be up and running then.

Large-group plans, however, are exempt from the requirement to provide the essential health benefits, now or in 2014.

But advocates say that companies and insurers should cover maternity care even if they’re not required to.

‘‘For young girls, this is a basic health benefit that they need,’’ said Debra Ness, president of the National Partnership for Women and Families.

“Why would they deny them access to a health benefit that’s so essential?’’