It could be the wave of immigrants flooding the city, or something in “the water.” Probably it’s related to post-9/11 personal growth.

Whatever the reasons, New York City is in the midst of a dramatic baby boom, according to census estimates to be released today. In Manhattan alone, the number of children under age 5 rose by 20% between 2000 and 2003, the Census Bureau estimated.

The Bronx saw an increase of 4.8% in that age group, the new study said. The population of the youngest New Yorkers rose by about 2.3% in Brooklyn and by an estimated 1.1% in Queens. Only Staten Island saw a decrease: In the smallest borough, the population of the smallest people declined by 6% between 2000 and 2003, the census study estimated.

For New York City as a whole, the estimated change in the population under age 5 was plus 4.6%. Meanwhile, the rest of the state was experiencing a baby bust. The overall population of New York in the 0-4 age group, according to census estimates reported by the Associated Press, dipped by 1.9%.

Opinions on the cause of this newfound popularity of procreation vary depending on whom you ask. But the boom’s impact in the coming years on New York City’s unique character and commitment-phobic culture, not to mention the city’s already overcrowded schools, will surely prove significant.

A political scientist at Columbia University, Rudy de la Garza, said the surge is probably centered in the city’s vibrant immigrant communities, where enclaves of Mexicans, Koreans, Chinese, and Albanians have matured into new neighborhoods that continue to grow.

“Immigrants are mostly young, working-age, in their 20s or 30s,” he said. “There’s no population as likely to reproduce. This is a place with a tremendous service industry, where people just starting out can get jobs. This city is so busy. You need a system to support it. Immigrants provide that.”

Other studies back Mr.de la Garza up on that. In the once-devastated Bronx alone, according to the census bureau, the overall population grew 11% to 1.3 million between 1990 and 2000, largely on the strength of newly arriving Mexicans, Dominicans, and Albanians.

The dramatic drop in crime in recent years, along with other improvements in quality of life, plays a significant role in the willingness of parents to raise small children here, experts and parents said. Still, some veteran residents of the city voiced simpler theories.

“I’ve been seeing a lot of pregnant ladies lately,” said a 44-year-old Brooklyn native, Greg Larocco, who manages a department store downtown stocked with clothes for infants and with Scooby Doo and Elmo backpacks. “Maybe it’s the water they’re drinking.”

A Falun Gong practitioner from Northeast China, Libei Bateman, offered a popular view: that the baby boom is related to September 11.

“When people are seeking peace in their hearts, they seek other people,” she said. “I think after this terrible incident people think more about their hearts, about relationships.”

It’s a theory that’s hard to dismiss. After all, World War II was followed by a baby boom that has shaped modern America. Not far from ground zero, at the Battery Park City Day Nursery, demand is now so high that the program recently expanded into two additional classrooms to accommodate more 2- and 3-year-olds and has a waiting list, according to its director, Denise Coraibano. “It’s crazy,” she said. “We went from 70 children in 1999 to 138 now.”

Uptown at La Escuelita, a bilingual program on West 91st Street that opened in September 2003, business is likewise booming. One of the school’s two co-founders, Jennifer Woodruff, said the nonprofit school has been experiencing better-than-projected financial results, pulling in $500,000 a year in revenue and enabling it to award its teachers raises of 10%.