Evidence is mounting that smoking and drinking mothers do a lot more harm to their babies than themselves.

Doctors speaking at the 146th annual meeting of the American Association for the Advancement of Science said today that heavy maternal smoking causes spontaneous abortion, still-births and as many as 35 percent of the premature births in the United States.

Doctors said that babies born to smoking mothers can be expected to be smaller at birth and grow slower and come down with more illness after birth than children born to women who don't smoke or quit smoking while pregnant.

"There are many unfavorable effects of smoking during pregnancy that are worse the more the mother smokes," Dr. Mary B. Meyer of the John Hopkins School of Hygiene and Public Health said at a symposium on fetal health. "It is quite clear that smoking increases the risk of abortion, stillbirth and preterm delivery."

Alcoholic mothers now account for the third most common recognizable cause of mental retardation in the United States, said Dr. Kenneth Jones of the University Hospital of San Diego. He said they give birth to retarded babies who now make up as many as two out of every 1,000 infants born nationwide every year. So serious is this problem that doctors have a name for it: the fetal alcohol syndrome, which describes a baby with shortened features, a pugnacious cast to its face and eyes placed too close together.

"It's associated with a very specific pattern of malformation," Dr. Jones said. "These children are so characteristic in the way they look thay you can often identify them from across a room."

While doctors say there is little doubt that maternal smoking and drinking is harmful to the babies, they admit to considerable doubt of why this is so.

Smoking appears to deprive the unborn child of oxygen, in part because nicotine causes erratic changes in the mother's blood flow and in part because the carbon monoxide inhaled by the mother binds itself to hemoglobin in the mother's and the baby's blood that reduces the amount of oxygen reaching the unborn baby.

"The carbon monoxide from the cigarettes occupies sites that would otherwise be bound to oxygen," Dr. Meyer said. "The problem is a worsening one, because in the past 10 years young women have taken up smoking in increasing numbers and older female smokers have tended to smoke more."

The results of heavy maternal smoking are staggering. Dr. Meyer said it can cause bleeding and premature rupture of membranes in the uterus. It also can cause premature separation of the placenta from which the fetus draws its nutrients. She said recent studies show that between 25 and 35 percent of the births of less than 32 weeks gestation and of birth weights less than 1,500 grams (4.3 pounds) can be attributed to maternal smoking.

"There is increasing evidence that babies of smoking women do not catch up to babies of nonsmokers in growth," Dr. Meyer said. "They also have other deficiencies, such as more frequent hospitalization, more visits from the doctor and longer durations of hospital stay, especially for respiratory desease."

While the connection between retardation and chronic alcoholism in the mother is now clear, Dr. Jones said there is evidence that even moderate or binge drinking by pregnant women can result in birth of babies who are less than healthy and normal.

"There are a terrific number of unanswered questions about this issue but it is one we need to study," Jones said. "It's a lot easier to study the offspring of chronic alcoholic women and of women who don't drink at all. but this issue in between, of moderate, binge or social drinkers is very difficult. My advice to pregnant womem is that if you don't have to drink during pregancy, don't drink at all."

Dr. Summer . Yaffe of the University of Pennsylvania Children's Hospital told the meeting of studies he and Dr. Chhanda Gupta had injected phenobarbitol into pregnant rats; the rats gave birth to offspring who turned out to be smaller, suffer hormonal changes, reach puberty later and have a higher incidence of infertility than rats born to mothers who were not given the injections. Most of the offspring who suffered the changes were female but not all.

The rats in the study were given small doses of phenobarbitol on the 17th, 18th and 19th days of the normal 21-day gestation period for rats. The dose was so small each day that it did not put the mother rat to sleep.

"Despite such a modest dose of what is one fo the most prescribed sedatives in America," Yaffe said, "fully half the animals born to the rats given injections were abnormal. More than half the female offspring rats were infertile, at least 40 percent suffered abnormal menstrual cycles when they reached puberty."

Yaffe said he does not know the mechanism causing such changes in rats born to mothers given phenobarbitol. He said he believes the drug somehow alters the brain at a key time just before birth when the rat's brain is in its final stage of development.

However it works, these animals do not revert to normal," Yaffe said. "Whatever the hypnotic drug does, it does it permanently."