It could be that the new generation of millennial women is delaying having children even longer than the women who came before them, as prime childbearing years are also critical years for advancing in a career. A recent study shows that the marital pay gap that springs up after a first child is born typically does not close if the birth happens between age 25 and 35.

Shannon Hettinger, a 32-year-old from Washington, D.C., said she definitely wanted children. She grew up in a large family in a small town in Pennsylvania and almost all her high school friends are married with children. But she moved to Washington, and spent her 20s deciding on a career. Now that she has one she loves — she works in residential real estate sales — she is not going to stop until she gets established. That means not having children for a while.

“I just want to build my book of business and see where I can go from here,” she said. “My whole focus is career growth. That’s my No. 1 priority.”

“Once I achieve a certain level of success,” she added, “then I’ll start thinking about a family.”

Ivy Gray-Klein, 26, who lives in Philadelphia and works at the University of Pennsylvania School of Design, said she was open to having children but cannot imagine doing so until she is 30 or 35. She wants to feel settled in her own life first. Now she has three roommates, is paying down her student loans and is working to build a little bit of savings.

“I’m just really trying to get myself to a place that is solid,” she said by phone. “Having a child right now would be so destabilizing. Children just seem like such an enormous financial undertaking.”

The country has had declines before. The longest period of continuous decline on record was from 1958 to 1968, according to Brady E. Hamilton, a statistician and demographer with the National Center for Health Statistics. The United States has been tracking the number of births and birthrates since about 1910.