Q: I am the mother of two girls, 3 1/2 and 14 months. I left work before the older one was born and have remained home with them ever since. I would eventually like to have another child but not until the younger one is 3. This way I can give her a lot of attention before she becomes "the middle child."

I have been thinking about our family becoming a foster family for a physically or mentally handicapped infant. Since I have chosen, for now, a career of full-time mothering, I feel I can offer something to an infant who may have no one else.

After evaluating whether I could handle the added stress, my remaining concern is the effect on my two daughters. Are they too young to have their mother devote this much time to an infant who may be with them a year or more? Would they accept this child as another sibling and what would be the effect when the child eventually has to leave? Would I be defeating the purpose of postponing the birth of our third child?

A: Your concern and generosity is heartwarming, but the wisdom of your idea is less than certain. This is the time to be realistic.

A normal, healthy baby takes a great deal of care, and a handicapped baby requires much more. While you may be ready for all the giving that's required, your daughters may not be -- especially the younger child. In just another month or so she will start to get more rambunctious. And more. And more. And your 3-year-old will soon be an out-of-bounds 4. During the whole year or so that you would expect to have the foster child, you would also be dealing with your own children at two of their most stressful ages. Not all 2-year-olds and 4-year-olds are as difficult as child development people (and most parents) report, of course, but even the most easy-going children are harder to handle when they are under stress.

Your children are sure to feel at least a little displaced when you give the foster child the love and attention that would be needed. In addition to the obvious time that requires, there would be the hours spent giving special therapy, waiting in doctors' offices and untying some of the red tape that comes with foster care.

Your husband may feel shortchanged, too. A father carries a heavy burden, especially when he is the only wage-earner. He should be in full agreement with this idea before it's undertaken, since a foster child would add to his worry, even though you'd get a small stipend and do most of the work.

The needs of the foster child must also be considered and, indeed, they are foremost. It wouldn't be fair to start the arrangement and then quit it ahead of schedule, but that's what you'd have to do if it was more than you and your family could handle. And it may be, since it usually takes longer to place a handicapped child.

You'll also want to consider the impact on the whole family. An addition -- or subtraction -- in a household will change its rhythm, and these changes aren't always for the best.

And finally there is the effect a foster child might have on you. The ambivalence parents sometimes feel about their well and contented children is magnified by a foster child, particularly one who is handicapped.

These are the negatives, and only you can decide on the weight they should have.

If you do decide to take a foster child on a long-term assignment, an older one, with or without a handicap, might be easier for you and easier for your children to accept. There is a tremendous need for foster parents around the country, especially for those who enjoy teen-agers.

Rather than the long-term care of a child, you also might consider going to a county agency, a religious service group or an adoption agency and arrange to have one or two of their institutionalized pre-schoolers spend weekends with your family. Or you could try short-term infant care -- being on call to handle a normal newborn for up to a month. Either approach would give you a taste of the foster experience, and it would keep you from getting too attached. Grief can be great when you part with a foster child, and the more you invest of yourself, the greater the grief can be.

If you do extend your family, in any way, you'll need a supportive social worker. Your children will need appreciation, too -- from you and their dad -- and they will also need you to be honest with them. The foster child is not a sibling and shouldn't be labeled as one. This would make the inevitable breakup much harder for your own children and make them worry if they would be the next to go. Instead, you would need to explain to your little girls that you are babysitting until the real parents can be found. This simple story, told often, will be understood -- and it will be the truth. Real parents are the ones who give a lifetime of care.

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