Take a walk on the Upper East Side or stop in a TriBeCa park and you may think you’re seeing double – that’s because the birth rate of twins in those neighborhoods is more than double the city average.

With the rising popularity of medical treatments for infertility, the rate of twin births in New York City has reached 3.8%, but from Kips Bay to Yorkville and from Battery Park to about 59th Street on the West Side of Manhattan, the incidence is closer to 8%, according to an analysis by the city Department of Health and Mental Hygiene for The New York Sun.

City demographers and experts on twin research say women in those high-income ZIP codes tend to be professionals who delay child bearing until later in life and are more likely to seek out the help of one of the city’s many fertility clinics.

“It’s almost like, ‘Who doesn’t have twins?'” Andrew Stenzler, the co-founder of Kidville, a high-end recreation emporium on 84th Street near Lexington Avenue, said about his clientele. The center catering to the 5-and-under set provides a special aisle for doublewide strollers, which is almost always full.

An Upper East Side mother of 9-year-old girls, Miriam Schneider, heads the Manhattan Mothers of Twins club. The group now has about 450 active members who stay connected with e-mail groups and monthly meetings about everything including how to get twins into preschool.

Ms. Schneider, 49, a banker, and her husband, 81, went to the fertility clinic at what is now NewYork-Presbyterian Hospital when they had trouble conceiving. She said about 60% of the mothers in her twins group had been treated for infertility.

“Where else except in New York can you look around and see the youngest mother in the room is 40?” she said.

A professor of political science and sociology at the CUNY Graduate Center, John Mollenkopf, said the increase on the West Side could be attributed to the swell of families taking advantage of real estate deals in Battery Park and neighboring areas after September 11, 2001.

Yelena Norasteh, 31, and her banker husband moved to Reade Street in TriBeCa just before having twin boys last year. She said she couldn’t believe how many sets of twins she sees; even her neighbors just had a pair.

The double-stroller with extra large wheels that she special-ordered from Germany after spotting a mother pushing one in Brooklyn has become the toast of the neighborhood.

“Everybody asks me where I got it,” she said about the buggy.

Nancy Segal, a psychologist who studies twins at California State University at Fullerton and the author of “Indivisible by Two,” a book on twins, said the rate of twinning has nearly doubled in America since 1980.

She said it is not surprising that the rate of twins is higher in some of the city’s wealthier neighborhoods, especially those with access to top-notch fertility clinics.

“Artificial reproduction techniques require a certain amount of money,” Ms. Segal said. In vitro fertilization can cost more than $10,000 a cycle and may have to be repeated if it is not successful; health insurance often only covers one cycle.

About 31% of women who use artificial reproductive technology give birth to twins, according to the Center for Disease Control.

“Many people have to try two, three, four times – so you’re talking about a lot of money here,” Ms. Segal said. Using an egg donor can cost anywhere from $2,000 to $25,000.

Black women are naturally more likely to give birth to twins, but because white women are more likely to use fertility technology, the numbers of twins among white and black women have recently evened out, Ms. Segal said.

At Bu and the Duck, a chic children’s shop in Tribeca with little Italian shoes and sweaters from Peru, the wall behind the counter is dotted with photos of twin customers – of which there are many.

“It’s basically because you have mostly professional women who wait to have babies and when it doesn’t happen in five minutes they run to consult a fertility expert,” said the store’s owner, Susan Lane. She said that her costumers tend not to buy matching sets for twins but rather ones that complement each other.

After giving birth to twin girls last year, real estate attorney Jody Googel couldn’t help bumping into other twin mothers on the streets of the Upper East Side. They would immediately exchange numbers and before long Ms. Googel was hosting dinner parties for about 13 mothers.

“I became the pied piper of twins,” Ms. Googel said. “I found that I didn’t want to be friends with that many people with single babies because they didn’t know what I was going through.”

Because twin mothers are less mobile than their counterparts – double strollers are almost impossible to take on the subway and can’t fit on buses – they tend to stick together in their neighborhoods.