Legal abortions in the United States increased 12 per cent in 1975, and one-third of the patients were teenagers, the national Center for Disease Control reported yesterday.

The CDC said 5 per cent of the teenagers, about 14,000, were 14 years old or younger.

The figures are contained in the CDC's annual abortion surveillance report, which this year gives figures for 1975. The report said more than 850,000 abortions were reported by the states in 1975, but those figures probably do not account for some abortions - perhaps an additional 15 per cent.

Using the reported figures, the ratio of abortions to live births was up from 242 to each 1,000 births in 1974 to 272 in 1975.

The abortion rate, measured for women between the ages of 14 and 44, rose from 17 abortions per 1,000 women in 1974 to 18 per 1,000 in 1975.

The redistribution of legal abortions into states which had restrictive laws before 1973 continued in 1975," the report said. Consequently, more women were getting abortions within their home states.

The study also shows:

Women obtaining legal abortions in 1975 tended to be pregnant 12 weeks or less at the time of the abortion.

A trend of declining deaths because of abortions, first noted in 1973, continued in 1975.

Sixty-five per cent of women undergoing legal abortions were white, while 31 per cent were non-white.

Only 26 per cent of women obtaining abortions in 1975 were married at the time of the procedure.

Forty-five per cent of those undergoing abortions had no living children.