So you're going to be a mother. You begin thinking about decorating the nursery, hiring a baby nurse, assembling a layette, and the high cost of college takes on meaning.

But there are more immediate costs - obstetrical care, which averages $2,600 nationally, and maternity clothes. Yes, your new profile merits a new wardrobe, and even if you do the smart thing and borrow a few garments from friends, it's no small expense.Judy Loeb, design director for Jeanette Maternities in New York, estimates a minimum $1,600 shopping spree for a working woman's spring-summer wardrobe. That's assuming your feet don't swell, requiring new shoes.

But it's cheaper than if you were buying regular clothes of comparable quality - 10 to 20 percent cheaper, according to Loeb.

"We're using the same fabric and cutting larger, but there's price resistance because they won't be wearing the clothes as long," Loeb says. "It's a problem for us because we're trying to deliver a quality product."

Loeb says that even though prospective mothers balk at spending on maternity clothes, they should buy what they look good in and feel good in because for six months those few pieces are going to be worn at least once a week.

"You wouldn't ask that of your regular wardrobe, would you?" she queries.

At any rate, Loeb says the most popular fashions for spring and summer are pants, particularly cotton, in both knit and bicycle styles. Two-piece dresses and slip dresses also are in demand.

Colors?

"Peacock, papaya and tomato with a lot of white," says Loeb.

The most needed items during the first three months of pregnancy are pants and skirts because of waist expansion, she says. The bulk of the wardrobe will be purchased during the second trimester, while purchases during the final three months will be items to boost the morale, plus nursing garments.

Of course, Loeb has a vested interest in maternity fashions, but she warns mothers-to-be not to buy larger sizes of regular clothing.

"The clothes are not cut to compensate for an expanding stomach," she says, "and will hike up in the front as pregnancy progresses."