Women should avoid alcohol completely if they are either pregnant or just "considering" pregnancy, the Reagan administration's chief doctor warned yesterday.

Dr. Edward N. Brandt, assistant secretary for health in the Department of Health and Human Services, issued a "Surgeon General's Advisory" stating this unequivocal advice and urged all doctors, in an individual mailing, to tell it to their patients.

The warning went even further than previous official statements, which have either urged women to curtail alcohol use during pregnancy or advocted abstinence as the "safest" course.

"Sizable and significant increases in spontaneous abortions" have been seen in pregnant women who drank as little as two ounces of alcohol twice weekly, Brandt said. There have also been "significantly decreased" birth weights in babies of some mothers who averaged an ounce of alcohol daily. Babies with decreased birth weights are more likely to have other, more serious problems.

Two standard or modest ounce-and-a-half "shots" of whiskey, gin or vodka contain more than a full ounce of pure alcohol (1.2 ounces of alcohol if the spirits are 80 proof, a bit more if they are stronger).

Brandt issued the "Surgeon General's Advisory," first of this administration, as acting surgeon general, a post he holds because the president's expected nomination of Dr. C. Everett Koop as surgeon general has been held up by House refusal so far to exempt Koop, who is 64, from this Public Health Service age limit.

The Brandt warning is follow-up to an issue raised in the Carter administration, when some scientists and doctors worried about the effect of alcohol on the fetus began urging a warning label on liquor, wine and beer bottles.

Last November a joint HHS-Treasury Department report recommended against a warning label, but said the government and the alcohol beverage industry should conduct a major campaign to educate the public on all alcohol dangers.

Brandt yesterday mentioned other practices, like smoking and poor nutrition, that can also affect fetuses. But alcohol's effects apparently are independent of these other dangers. When drinking is "heavy," he said, there is greater risk of outright birth defects, and when the expectant mother is an alcoholic, there is a risk of fetal alcohol syndrome, a grim cluster of severe physical and mental defects.

Alcohol consumption is especially harmful in pregnancy's early months, even in the earliest stages when a woman may still be unaware she is pregnant. This is one reason Brandt warned women even considering pregnancy to abstain.

Brandt urged doctors and other health professionals to ask all pregnant patients, or those planning pregnancy, about drinking habits. He pointed out that alcohol is found in some drugs and foods -- vanilla and some desserts, for example -- and said doctors should help patients be aware of these, too.