One-parent families now account for 26 percent of all families with children under 18 in the United States, compared with 13 percent in 1970, the Census Bureau reported yesterday.

The swift rise of the one-parent family has been one of the major social developments of recent times, seen by many students of social trends as a major cause of poverty and welfare dependency.

Nine-tenths are headed by women and they are disproportionately black and poor.

There is no universally accepted explanation of why one-parent families have grown from a small fraction of the population to more than one-quarter of all families with children, but among the reasons usually cited are the sexual revolution, resulting in more births out of wedlock; wider job opportunities for women; increasing divorce rates; the poor economic prospects of young black males, and the availability of welfare for girls who become pregnant without being married.

Whatever the explanation, the Census Bureau survey, taken in March 1984, shows that the one-parent phenomenon has increased greatly among whites and blacks.

In 1984, the survey found, there were 33.2 million families with children under 18 in the United States, and 8.5 million of them were one-parent families -- 26 percent, compared with 22 percent in 1980 and 13 percent in 1970.

Among white families with children, the proportion of one-parent families was 20 percent, compared with 10 percent in 1970; among black families, it was 59 percent, compared with 36 percent in 1970.

Although the poverty rate for the nation as a whole was 15.2 percent in 1983, it was 40 percent for single-parent families headed by white women and 60 percent for those headed by black women.

Of 7.6 million women of all races heading one-parent families, 2.1 million were never married, 1.8 million were married but the husband was absent, 3.2 million were divorced and about a half-million were widowed.