Most women who take birth control pills have higher levels of the blood fats that most doctors think help bring on premature heart attacks.

That was a finding of a study of pill-using women at 10 American medical centers, doctors at these centers and the government's National Institutes of Health have reported.

The findings may help explain why women on the pill, especially women who also smoke and therefore are at still greater risk, are more subject than other women to heart attacks and other blood vessel problems.

There have been previous reports on the levels of blood lipids - fatty and waxy products of the body's chemical machinery - in oral contraceptive users. Some have said levels were higher; some have not.

Doctors at the 10 centers' Lipid Research Clinics, financed by NIH's National Heart, Lung and Blood Institute, studied 2,606 women, a large enough group for statistically significant findings.

As reported in a lead article in the British journal, Lancet, and explained further by the heart institute's Dr. Basil Rifkind:

Pill-users had increased levels of total cholesterol, LDL (low-density lipoprotein) cholesterol and triglycerides. LDL cholesterol seems to be cholesterol's most harmful fraction. Triglycerides are other fats.

The increases ranged from 1 percent to 10 percent for total cholesterol and LDL colesterol, and around 50 percent for triglycerides.

Younger pill-users, women in their 20s, tended to have larger differences in blood fats compared with other women of the same age.

Paradoxically, older, post-menopausal women taking estrogens, or female sex hormones - chemicals like these in the birth control pills - to combat some symptoms of menopause had lower, rather than higher, levels of total cholesterol, LDL cholesterol and triglyceride than others their age.

They also had higher HDL (high-density lipoprotein) levels. A high HDL cholesterol level may help protect a person against heart attack.

But these findings about older women and estrogens were accompanied by a double warning.

There is still no proof that blood lipids, plentiful or sparse, cause or protect against heart attacks, though the evidence linking blood fats to higher incidence of heart attack is strong enough to make most authorities recommend a "prudent" low-fat, low-cholesterol diet for persons of all ages.

There is much firmer evidence that older women who take estrogens for an extended period of time have a higher-than-normal incidence of cancer of the uterus. Most authorities now think these drugs shouldn't be used during menopause unless there is clear medical need.

The overall study was based on oral contraceptive use between 1971 and 1976. Most women studied were taking pills with more estrogen than most pills used today. This means the levels of possibly harmful blood fats might be somewhat lower among pill-users today.

Dr. Daniel Mishell, University of Southern California obstetrics chairman, recently said "most new prescriptions" are for pills with 30 to 35 micrograms of estrogen, which provide "efficacy, safety and [a] generally low level" of side effects.

But a "surprising 27 percent of pill use in the U.S.A. is still in the high-estrogen category," a Lancet editorial said in May.

Like many, though by no means all, obstetricians, Mishell called the pill's risks "very small" in women under 35 who don't smoke or have other conditions predisposing them to heart or blood vessel disease, such as obesity or high blood pressure.

Not all authorities call the risks "small." In plain numbers, according to Boston University epidemiologists, the heart attack risk for a normal woman who neither smokes nor uses the pill gradually increases each year from nearly zero in her early 20s to 10 chances in 100,000 by her early 40s and 20 in 100,000 by her late 40s.

The risk each year is multiplied by four if she uses the pill. It is multiplied by 40 if she smokes as well.