Despite the great recent drop in the birth rate and greatly increased use of birth control methods, more than two births in five are still unplanned or unwanted, even among married couples, a new survey indicated yesterday.

The figure means the number of births could still be reduced drastically - by more than half a million of the nation's 3.1 million births a year - if all unwanted and all out-of-wedlock conceptions were eliminated, said one of the survey's authors, a family planning official at the federal Center for Disease Control in Atlanta.

The U.S. fertility rate - or total number of children borne by each woman aged 15 to 44 - has been 1.8 for three years. This is well below what population scientists call the "replacement rate" - the rate that would keep the population just level - of 2.1.

But fertility remains much higher among the poor, the young and the unmarried, the survey showed.

Also, it indicated, only 56 per cent of children born to married couples are planned. Fourteen per cent are unwanted, and another 27 per cent are "mistimed," meaning the couple did not want a child at that particular time, the study said. A few parents said they did not know how they felt about having their child.

Among unmarrieds, 19 per cent of births were called "planned" by the mothers, 7 per cent were called unwanted and 74 per cent were described as mistimed - in this case usually meaning conceived out of wedlock by a couple intending to marry.

Federal and New York state health officials made the survey by interviewing a sample of 2,059 women in 18 counties of upper New York state, including the cities of Albany, Troy and Schenectady. Theirs was the first U.S. study to examine the fertility status of both unmarried and married women of all reprodcutive ages.

The results are generally consistent with figures in nationwide studies, said Leo Morris, an assistant chief in the Disease Control Center's Family Planning Division.

The survey showed that unwanted and unplanned births "remain a tragic fact "for hundreds of thousands of individuals and couples a year, said Dr. Louise Tyrer, medical vice president of the Planned Parenthood Federation.

The survey results appeared in Family Planning Perspectives, Planned Parenthood-World Population publication.

Besides Morris, the authors are John E. Anderson of CDC and Melita Gesche, New York state family planning director. They said the results show "continued need for publicly funded programs to prevent unplanned childbearing," especially for those who still lack effective contraception.

They found that women in higher income brackets reported 62 per cent of their births as planned, compared with 33 per cent among lower income mothers.

Black women had a slightly higher fertility rate and somewhat fewer planned births than whites, but greater differences were based on economic status. Poorer women have more unplanned and unwanted births "regardless of race," the authors said.

They found least use of contraceptives among the poor and among sexually active, unmarried teenagers. Almost six unwanted births in 10 were from women who used no contraceptives.

The Census Bureau reported yesterday that the current fertility rate of 1.8 children per woman is just half the nation's 1957 level.

As a result, the bureau said:

There are now 7.6 million fewer schoolchildren aged 13 and under than there were in 1970, though there are more women of childbearing age.

Between 1970 and 1976, the preschool population (children under 5) fell by 1.8 million and the elementary school group (5 to 13) by 5.8 million, both dropping 10.6 per cent.

The median age of Americans has risen to 29 years.

Also yesterday, the Washington-based organization, Ezro Population ZGrowth, issued a Vlenniet Day's reminder aimed at teen-agers that said:

"Love carefully. Worship Cupid but don't be stupid. Teen-age pregnancy is no sweetheart."