Three-year-old Christian Diaz made his first visit to the dentist and didn't cry or squirm or fidget one bit in the big green chair. But then he wasn't far from home.

The dentist, working out of a 30-foot van dubbed the Mama & Baby Bus, came right to the street around the corner from his Northwest Washington home.

Outside, hip-hop music throbbed from the speakers of the Good-To-Go Thrift Store while folks scurried out of the snowflakes into Johnny's Carry-Out and the check cashing store. Inside the van, the dentist, Candace Mitchell, labored away.

"Okay, Christian, I'm going to clean your teeth," she said, adding in Spanish: "No te duele; suave." It doesn't hurt; gentle.

To which the wide-eyed child replied: "A ver si me hace cosquillas." Let's see if it tickles me.

And his mother, Marina Cruz, chimed in. Laying her hand gently on his head, she said, "Habre la boca; habre la boca grande." Open your mouth; open your mouth wide.

This was the semi-monthly visit of the Mama & Baby Bus to the Petworth neighborhood in Ward 4, an area that has no free clinic or hospital, a burgeoning population of Latino, African and Caribbean immigrants and pockets of some of the city's poorest residents next to some of the wealthiest. But one of the reasons for the bus is that the District through the 1990s, when ranked with the states, was at the very bottom nationally in three key perinatal indicators: adequate prenatal care for women (50th), the number of preterm babies, or those of born before the 37th week of pregnancy (51st) and infant mortality (51st).

"So what's the best way to get past the race issue, the gender issue, the age issue and the class issue? Deal with health," said Ray Michael Bridgewater, president of the Assembly of Petworth, a grass-roots group in Ward 4 and a member of the community advisory board for the Mama & Baby Bus.

That's what the March of Dimes, along with Mary's Center for Maternal and Child Health, a clinic in Adams Morgan, has been doing for three years with the bus. The mobile unit contains two small offices, one with a dental chair and dental equipment and the other with an examination table.

The project was proposed in 1999 by the National Capital Area Chapter of the March of Dimes. "But we felt the problem in Washington was well beyond the scope of our chapter," said Dona Dei, director of program services. "So we went to our national office and board of directors and they said we would like to help you by giving you more money to get a vehicle."

The bus was launched with a $340,000 grant from the national March of Dimes. Since then, the D.C. chapter has donated $200,000 yearly to keep the service running.

The March of Dimes originally tapped the Public Benefit Corp., a quasi-governmental agency that ran D.C. General Hospital and a network of public clinics, to operate the bus. But after a year, in 2001, the PBC failed financially and was disbanded. Mary's Center, which serves more than 5,200 primarily Latino women and children yearly, took over operation of the bus to target some of the District's neediest and most underserved areas. It visits two sites each in Ward 1 and Ward 4 and one site each in Ward 2 and Ward 5.

The main goals, said Ken LaOrden, clinical director for Mary's Center, are to provide primary dental care and health screenings and primary prevention services to uninsured and underserved women and children. Clients are then directed into a "medical home," either a public clinic or the hospital closest to their residence.

"Mobile health care should not be a primary medical home. We just want to be the link," said LaOrden. "We need to get [needy people] into the system."

The staff includes a registered nurse or pediatric nurse health practitioner, a dentist, a case manager and bilingual outreach workers. The van offers pregnancy testing and family planning; prenatal testing and referral; immunizations; examinations and testing for sexually transmitted diseases; HIV testing and counseling; dental screening; Medicaid enrollment; preconception exams and postpartum depression screening.

That's not all. The case manager and health educator on the bus, Angelica Castillo, does a lot more than dispense health advice. She encourages Latino parents to get involved in their children's schools and helps families find adequate housing, especially Hispanics, many of whom live six to a studio or eight to a one-bedroom apartment because they cannot afford the District's high rents.

She said it took her weeks to find city-run transitional housing for a woman from El Salvador who arrived in the District in 2000 and was essentially homeless by the next year, living from friend to friend with her four children. The situation was so disruptive that her children were in school only two days a week. The woman now has a job, and her children are in school full time and know English. They're about to move into their own apartment, and they are all enrolled in a health clinic.

"Once they feel confident in a clinic, they begin to work with the social worker and I can leave them," Castillo said. By being on the bus, she said, "I'm the bridge."

The same day Christian got his his first teeth-cleaning, Castillo played that same role for Ermitania de Leon, 23, who showed up at the bus as a "walk-in" with her 2- and 3-year-old daughters. The bus was now packed. The dentist worked on one of de Leon's daughters, while the nurse health practitioner administered an HIV test behind a closed door to another patient. De Leon needed to sign up for the D.C. Healthcare Alliance, and Castillo was talking to her about Head Start for her girls. She has a son in kindergarten.

She wants insurance, the Guatemalan immigrant said, so that she can succeed in her family planning efforts and get back to work -- maybe even into a class to learn how to read and write, not only in English, but in her native Spanish as well.

"Everything here is very nice," she said about the bus. "You can talk about all your problems."

LaOrden said that since Mary's Center took over the bus, workers have given more than 500 screenings to patients and have followed up on all the prenatal cases they have seen. They have conducted more than 500 dental checkups and cleanings, more than 50 pregnancy tests and more than 140 HIV tests. So far, 600 children have been vaccinated, while more than 115 education sessions were given regarding folic acid, oral health, family planning and how to prevent sexually transmitted diseases or HIV infection. In addition, more than 850 people have signed up for one of the District's health insurance programs for low-income people and undocumented immigrants, either the D.C. Healthy Families program or the D.C. Healthcare Alliance.

The Mama & Baby Bus is the only health care van in the District that focuses on pregnant women and children and that offers dental care. It is one of 14 mobile units that operate in the city, most of which try to let residents know about health care services. The mobile units offer a range of services, from health care for the homeless to lead screening for children to mammograms for low-income women. There are also two pediatric care mobile units operated by Georgetown University Hospital and Children's Hospital.

"We all work . . . to complement one another," said Matthew Levy, medical director of community pediatrics at Georgetown University Hospital and a member of the community advisory board for the Mama & Baby Bus.

"There's a disparity of resources here," he said. "Where the services are most needed, there are the fewest resources. Mobile health is exactly what's needed in this climate, although it should only be a temporary solution."

Monica Reyes waits for her mother Ermitania de Leon, right, to finish talking with the bus's health educator, Angelica Castillo.Dentist Candace Mitchell shows Monica Reyes how to brush her teeth aboard the bus, which is run by Mary's Center for Maternal and Child Health and the March of Dimes. The bus helps provide primary dental care and screening. Below, Nancy Reyes gets an ear exam.Case manager and health educator Angelica Castillo calls clients waiting to see a doctor or dentist on the bus.