SAN ANTONIO — Every three minutes, a child is born somewhere in Texas.

At one hospital in North Texas, 107 babies were delivered over 96 hours this summer, shattering local records. At a hospital in San Antonio, more than 1,200 babies have been born this year, up nearly 30 percent since 2018.

Across one of the nation’s fastest-growing states, an average 1,000 new Texans arrive every day. Half of them are newborns.

“Our population is going up. So just with that, I would expect our birthrates to increase,” said Shad Deering, a department chair with the Children’s Hospital of San Antonio. “We will become very busy.”

We spent a day last month with Dr. Deering and his staff and witnessed the arrival of several new residents to the Lone Star State.

Stefanie Garcia-DeLeon was eager to hear her newborn when the machines she was connected to signaled something was not right. More than a dozen men and women in scrubs rushed into her room.

But nine minutes later, all worry evaporated. Ms. Garcia pushed, and a little girl let out a hearty cry. The room erupted in cheers and laughter.

Ten minutes later, Serafina’s little eyes scanned her mother’s face and all of the hospital equipment around her. “She’s so small, but already she has a big curiosity,” Ms. Garcia said.

Across the state, a baby boom has been fueled by newcomers from states like California and New York, attracted by a lower cost of living, less crowded schools and cheaper taxes. Many of them are starting their own families in the process, experts said.

“We have a higher proportion of population in the reproductive years,” said Lloyd Potter, a state demographer and professor at the University of Texas at San Antonio.

Between 2010 and 2020, the state’s population grew by four million — or the entire population of neighboring Oklahoma. Babies made up the largest number of new arrivals to Texas (about 48 percent), with migrants from other states (31 percent) and countries (21 percent) rounding out the rest.

And hospitals are trying to keep up.

“It has not slowed down,” said Michelle Stemley, vice president of patient care at Baylor Scott & White All Saints Medical Center in Fort Worth, which broke its four-day delivery record this summer.

The surge in births comes amid a declining birthrate nationwide. Couples have waited longer to have children, a trend that continued during the coronavirus pandemic and an uncertain economy, Mr. Potter said.

But a spike in sales of pregnancy tests — a 13 percent increase since June of last year — may signal that a so-called millennial baby boom may be on the horizon, according to Nielsen’s data and Bank of America’s research.

Many longtime Texans are contributing to the uptick in tiny new residents.

Amanda Ramos, 32, a mother to two children, was not expecting to have a third. Ms. Ramos was using an intrauterine device as birth control when doctors told her she had an ectopic pregnancy, which occurs when the fertilized egg implants in tissue outside of the uterus.

After surgery to remove the left tube of her uterus, she learned she was still pregnant. Eventually, at Children’s Hospital, Mateo Chris was born via C-section after doctors discovered he was pointing in the wrong direction and could not be born naturally.

Ms. Ramos held tightly to her baby, whose first name means “God’s blessing.”

“He’s a miracle baby,” she said. “My miracle baby.” 

Oliver Noteware, 34, and Kathryn Adkins, 33, grew up in Houston and attended the same elementary school. They fell out of touch for two decades but reconnected in New York, where they ran into each other on the streets of downtown Manhattan.

As the coronavirus tore through the city, they decided, like thousands of others, to try their luck elsewhere. They settled in San Antonio, where Southern manners has made it easier for them to meet new people. “We actually know our neighbors here,” she said.

On this day, Esmé Tallulah, all 7 pounds 7 ounces, joined them.

Mr. Noteware, who was also celebrating his birthday, held his sleeping daughter on his exposed chest to bond with her, as an exhausted Ms. Adkins watched with tenderness from her hospital bed. After 12 hours of labor, she was more than elated. “I feel a lot more connection to her now than I did when she was in the womb,” she said.

Both Mr. Noteware and Ms. Adkins said they were ready for the challenges of parenthood. When their baby let out a faint cry after a nurse pierced her tiny foot for a blood sample, the new mother did not miss a beat. “You are going to experience a lot more pain in life,” Ms. Adkins said with a bittersweet smile.