Q. "A couple of weeks ago you wrote about the joys -- and the woes -- of having a baby," writes an Arlington woman.

"That's all I ever seem to hear about. Or I hear about all the things that could go wrong if I get an anesthetic (or if I don't) or if I breast-feed or if I don't).

"I don't have a lot of hang-ups about these things. I'll take my Lamaze classes, and if I can have the baby without a pain-killer, fine, and if I can't, that's okay too. And I'll breast feed if I can, but if I can't I'll use a formula and be thankful it's available.

"What I want to know are the practical things. How big should a layette be? What equipment do I need? What do I take to the hospital? What can I call on the grandmothers for? (And why am I willing to take advice from a stranger before I listen to my own mother-in-law -- or my own mother?"

A. As you intuitively know, parenthood, like everything else, works best if you know what to expect, what can be solved, and what, somehow, must be endured.

Just being out of groceries when you get home from the hospital can make you feel like such an incompetent wife you start thinking of yourself as an incompetent mother. For convenience, efficiency -- and self-esteem -- stock up on all kinds of grocery and cleaning supplies in your last weeks of pregnancy. It not only gives you a satisfying job in those last impaitent hours before labor begin (when the nesting urge has you rushing about), but it's a lot eaiser to stock shelves than scrub floors.

For equipment, you need a crib for the first few months, a cradle, but if you live in a house with more than one floor you need a carriage, a bassinet or bureau drawer on each level to make things easier for you. You also need a umbrella stroller, an infant seat, a car seat and if you possibly can, a doorway swing, for when a baby is awake, he wants something new to look at every half-hour or so. Who wouldn't.

As for the layette, buy only practical things for your child if you have a lot of friends and family. The sweaters and bootees, the bunting bibs, the hats and the pretties are usually what you get for presents. Instead, put your money into four dozen pre-folded diapers (or half as many, if you're going to use diaper service or disposable diapers), three crib sheets, a blanket, some extra pillow cases for the bassinet mattress, six cotton pads and six flannel ones, a half-dozen undershirts, jumpsuits and kimonos, and a couple of waterproof pants (you want to see if he can tolerate the plastic before buying any more).

You need much less supplies than you might think. You only need petroleum jelly -- the best protection a baby can have after he's changed -- some cornstartch in case he gets a rash, a jar of cotton ball and water, to wash his bottom. You aslo should get a rectal thermometer. If you think your baby is sick, you can bet it will be in the middle of the night and the doctor will ask you if he has a fever. You don't want to guess.

The hospital suitcase takes planning too, even for a two-day-stay.

A well-packed one -- and it took us four babies to find out -- should contain much more than a couple of glamorous nightgowns, a bedjacket, a robe, a going-home outfit for the baby and for you a blouse, a wrap-around skirt (that will barely wrap) and a couple of nursing bras.

What you really need is perfume, body lotion, nail polish and makeup. It's going to be a while before you have time to pamper yourself again.

Also pack the birth announcements, stamped, addressed and ready for the statistics; an address book and a small gift in case you have a nurse you particularly want to thank.

The list goes on: a loaded camera, some extra rolls of film and some flash bulbs, (no, glare won't hurt the baby's eyes) and a couple of books, if only to remind yourself that the most exciting plot is trivial when you have such a beautiful baby to watch.

And if you think the suitcase is a little heavy, that's not all. Pack a split of champagne to be chilled in a pitcher of ice, so you and your husband can launch this child in style. Even though nursing mothers can't have much alcohol, babies are meant to be celebrated.

We even learned to pack crackers, cheese, sodas and sherry for company, since visitors somehow have little to say after the first few comments and you find yourself a hostess. Remember: Only another Lamaze parent or the new grandmothers will be interested in your step-by-step account of the delivery.

There is a great deal those mothers of yours can tell you, and do for you, from running errands to running your household when you get home. But don't count on them for much unless you establish a good set of ground rules, so they remember who's in charge.

The best advice we've seen (and almost the only advice) had just been published in a slim paperback called "Congradulations! You're Going to be a Grandmother" by Lanie Carter (Oak Tree Publications, $3.95). Although it's a small press your bookstore can order it from a local distributor.

For yourself, we recommend "What Only a Mother Can Tell You About Having a Baby" by K. C. Cole (Doubleday & Co. $10.95). It's a compilation of the experiences of more than 200 women which should teach you a lot about what to expect and what you can do about it. The scope of this book may seem narrow, but to a pregnant woman or a brand new mother, every detail seems vital. And indeed it is.