WASHINGTON (AP) _ When Mom goes back to work, the job of caring for preschoolers most often falls to someone outside the family.

``I was stunned that the use of organized facilities went up to 30 percent. That's an all-time high,'' Census Bureau statistician Lynne M. Casper said of the growth in day care and preschool facilities.

In addition to the 30 percent of children in organized facilities _ up from 23 percent two years earlier _ 22 percent of children are cared for either in the home of a nonrelative or by a baby sitter, according to the report ``Who's Minding Our Preschoolers?''

That means under half _ 48 percent _ of the 9.9 million preschoolers counted in 1993 are watched by relatives while their mothers are on the job. That's down from 53 percent two years earlier.

The figures for 1993, the most recent available, come just days after the National Institute of Child Health and Human Development reported that day care doesn't hurt children's attachment to their mothers. The institute said that 15-month-old children taken care of by relatives or put in day care centers maintained secure bonds to their mothers.

When Jeanette Bayliss of Bowie, Md., returned to work after having her first child, now 6, grandparents helped out from time to time. But for day-to-day care, organized help was needed

She chose the day care center at the Pentagon, where she works, but wasn't satisfied. So when her son, now 4, came along, Mrs. Bayliss sent him to a licensed day care provider, who watches children at her home.

``That worked much better because the group is much smaller,'' she said. ``You don't have all those colds that you have with a center.''

As a bonus, it cost $35 a week less. ``It was quite a saving and it was a much better situation, too,'' she said.

Now, both youngsters are in Montessori schools, which she says is even better. Before, she said, ``I felt like I was paying for them to play all day.''

For one period, Mrs. Bayliss' husband, a policeman, went on a night shift while she worked days, and that permitted them to alternate watching the children.

That also fitted them into the Census finding that care by fathers shot up between 1988 and 1991 and then fell.

The increase from 15 percent to 20 percent of children being watched by fathers while mothers worked sparked some speculation of a growing social trend of dads becoming more involved with the families.

But the share plummeted back to 16 percent from 1991 to 1993 and, looking back, Casper says she now thinks both changes were economic rather than social.

At the time the share of father care was rising, there was a recession with more dads becoming unemployed or working only part-time _ meaning more fathers were available to watch the kids and less money was available to pay for care.

Then the economy improved, fathers went back to work, and the kids went back to organized day care.

Reversing the Bayliss experience, however, Casper pointed out that care of children by nonrelatives, either at home or the caregivers' home, has been declining steadily in recent years.

She speculated that news reports of nannies and baby sitters abusing children, some caught on hidden cameras, may have been a factor in this decline.

Other findings of the report, based on Census' Survey of Income and Program Participation:

_Families in the suburbs were more likely to use organized day care (32 percent) than families in central cities (28 percent).

_About four in 10 children in black and Hispanic families were cared for by grandparents or other relatives, compared with about two in 10 white children.

_Low-income families were more likely to seek family help than those above the poverty line, 60 percent to 46 percent.

_Women working day shifts were more likely to need organized child care than those on nights.