I'm very concerned about the birth of my third child, due in about two months. My other two children, who are twins, will only be 19 1/2 months old.

I know the birth of a sibling is always stressful, but this seems worse because the children are so young. Also, I've been home with them since they were born.

The children have a good relationship with their father, but he will be with me when I'm giving birth. My mother-in-law will stay with them, but the children only see their grandmother about once a month, so they're not very familiar with her.

I'm afraid this separation of two to five days will be very traumatic for them. They are not yet very verbal, so I don't think I can explain to them what's happening. What can I do to help them?

The separation won't be as hard on your children as you think, although 19-month-old twins may be pretty hard on your mother-in-law. They'll all be more comfortable if she can get together with them more often during the next two months. Maybe your husband can take them to visit her while you have a rest.

The new baby will stress the twins, but they'll probably adjust better than a single child could, because they have each other. And though they aren't very verbal yet they do need to be told about the baby now and told often. You don't want them to wonder what you'll bring home every time you go out for a quart of milk.

They'll know what to expect better if you show them The Amazing Newborn by Dr. Marshall Klaus and Phyllis H. Klaus (Addison-Wesley; $10.95), with its photographs of the dramatic responses a baby shows in the first 10 days. And they'll feel more a part of the event if you have them talk directly to the baby as they pat your tummy.

To prepare for your hospital stay, tape yourself singing the lullabyes and reading the stories you go through at night, so your mother-in-law can play it at bedtime. She'll also need clear, written instructions, including a list of quirks and fears the children may have.

Once you're in the hospital, phone the children often but try to call at their best time of day, so they (and you) will be less likely to get upset.

They'll get used to the idea of a new baby better if they have some concrete examples to prop on the mantel, like Polaroid pictures of you with the baby, their dad with the baby, and, if they can visit you at the hospital, a snap of all of you together.

And by all means, buy a couple of inexpensive, indestructible rubber dolls and pack them in your suitcase. They're to carry into the house, one for each child: presents from the new baby.

Although some jealousy is inevitable, the children won't be too aggressive or overprotective -- the flip side of aggression -- if you encourage them to entertain the newborn by talking, making faces and dancing around the bassinet. These loving kinds of help will let them feel involved but keep the baby safe.

Extra attention usually keeps most serious jealousy in check. Get a sitter for the baby while you and your husband take the older children out for a treat, then separate for a while, each taking one of the twins. Individual attention is good for any child, especially when there's a new baby.

This juggling act won't be easy, but it will be possible and even fun if you take care of yourself.

To do this, you've got to have some help in the next year. Join a baby-sitting co-op; find an older neighbor to sit for pay; or leave the children one to two times a week at a Mother's Day Out program. Grown-ups, and grown-up activities, will give you the respite, and the perspective, to enjoy your children much more.

Questions may be sent to P.O. Box 15310, Washington, D.C. 20003.