Virginia joins the District and Maryland tomorrow in requiring motorists to wear seat belts and in banning the use of phosphate detergents.

Those are among a handful of new laws that take effect Jan. 1 in Washington area jurisdictions.

The Virginia seat belt law, like those already on the books in the rest of the region, can be enforced only if a motorist is stopped for another offense.

Also like the Maryland and District laws, only drivers and front-seat passengers are obligated to wear the restraints.

Virginia State Police spokesman Charles Vaughan said, "We plan to enfore the law enthusiastically," with no warnings given. Infractions call for a $25 fine, but because violating the law is a civil offense, conviction does not add points to a driver's license.

Virginia is the 32nd state to enact some form of a seat belt law, according to a spokesman for the American Automobile Association.

A state police spokesman estimated that as many as 200 lives could be saved in Virginia in 1988 if up to 75 percent of Virginia drivers obey the law. About one-third of the state's drivers have voluntarily been using seat belts, according to Lt. Col. C.M. Robinson.

Of 721 drivers or passengers killed on Virginia highways through the end of October, only 71 were wearing seat belts, Robinson said.

Although the seat belt law is expected to save lives, state police say, an increase in the speed limit from 55 to 65 miles an hour on certain rural stretches of interstate highways in Virginia could increase highway deaths. The legislation to increase the speed limit will be introduced in the Virginia General Assembly in mid-January.

Most laws adopted by the 1987 sessions of the Virginia and Maryland legislatures took effect earlier this year, as did legislation approved by the D.C. Council.

One new Maryland law requires doctors to inform women about the "advantages, disadvantages and risks" of breast implantation before performing the procedure. The legislation also directs the Maryland Department of Health and Mental Hygiene to prepare literature "in laymen's language" that describes the side effects and warnings about such operations.

Another new Maryland law raises the maximum amount of small claims action that falls within the jurisdiction of the district courts from $1,000 to $2,500.

There are no new laws in the District taking effect tomorrow.

In banning the use of phosphates, Virginia is joining Maryland and the District as part of an attempt to clean up the Chesapeake Bay. Environmentalists say phosphates and nitrogen (not covered by the ban) feed the growth of algae, which rob aquatic life of essential sunlight and oxygen.

The law prohibits the sale or use of laundry detergents containing more than one-half of one percent phosphorous. Dish-washing detergents may contain up to 8.7 percent phosphorus.

Virginia's 28 inspectors of weights and measures will make spot checks to catch violators, but "we're not looking for any prosecutions on the first go-round," said Jim Lyles, chief of weights and measures for the state Department of Agriculture and Consumer Services. "It'll probably be a warning."

Lyles said the department had been getting the word out to retailers, packers and manufacturers for six months. He said preliminary checks of supermarket shelves indicate that phosphate detergents "are pretty much being eliminated."

The penalty for selling phosphates is $1,000 and a year in jail; users face only a fine.

Also in Virginia, a law that is supposed to ease a malpractice insurance crisis for obstetricians takes effect.

The Virginia Birth-Related Neurological Injury Compensation Act already has achieved its objective, according to its chief sponsor, Del. Clifton A. (Chip) Woodrum (D-Roanoke). Woodrum said 10 days after the law was enacted last spring, insurance companies lifted a ban on writing malpractice policies for Virginia obstetricians.

The law removes certain catastrophic birth defects, estimated to affect perhaps 40 births a year in the state, from the threat of lawsuits, by creating a fund from which parents will be compensated. The money will come from participating physicians and hopsitals.

Another new Virginia law gives prosecutors the right to appeal certain pretrial motions, but not acquittals.