Don’t worry about your question being long. From my standpoint, it is a lot easier to offer a helpful answer if a lot of information is given than if only the sketchiest details are sent. However, there is one critical piece of information you didn’t supply—how much she talks to you. Since you indicate a number of impressive verbal achievements—counting, knowing colors (though you didn’t clarify whether she says the names of the colors or merely points correctly)—she must talk to you at home. But knowing how much and under what circumstances would be helpful.
Even without that information, I have a few suggestions you might try:
(1) Make certain her hearing is OK. As you have discussed the situation with her doctor, I assume that this has been done. You may never have thought about this, but it is far more difficult to understand children than it is to understand adults. (If you doubt the accuracy of this statement, just teach a group of 20 4-year-olds for one day and count how many times you have to ask various ones to repeat what they have said.) This is as true for children as it is for adults. This adult-child discrepancy in understandability probably explains why first and only children tend to talk earlier than later children—they have more adult speech directed to them. The voices of little children are higher in frequency, and hearing loss at the higher frequencies is more common than at the lower frequencies typically used by adults.
(2) Talk to her a great deal, and make it easy for her to join you in conversation. Ask questions that can’t be answered with a nod or a gesture: “Which dress do you want to wear?” You can think of many other examples. Try not to ask too many questions that call for an informational answer (“What number comes after 19?”) and concentrate on those that allow a conversational give and take. Knowledge questions often generate so much anxiety that they inhibit a verbal response. Encourage her to tell you about things that happened at school, but avoid asking “Did you talk to anyone today?” Let her take things to school for show and tell, but minimize the “tell” component. “Would you like to take your new doll to school and show it to the children?”
(3) Do everything you can to reduce what may be a high level of anxiety in your daughter. If her cessation of talking was as abrupt as you describe, some traumatic event may have occurred in her childcare. I’m not suggesting that it did, but I wouldn’t rule it out. I wouldn’t try to get her to talk about it, but I would be alert to note whether she gives out any signals in her play that might suggest something happened to cause her to become fearful. If so, she might not even remember the event, but her play might give out hints. If so, “playing through” the event will be helpful to her.
(4) Invite another child to your house to play with your daughter. Leave the two alone to play with their toys, without any admonition, “Now you have to talk to Chrissie or she won’t want to come back and play with you.” Ideally have some housekeeping toys and dress up clothes they can play with. That kind of play calls for dialogue, and it may help your daughter to “play her role.” Do this as often as you can manage it—but only with one child at a time. If you invite two, they may talk just to one another and allow your daughter to remain outside the conversation.
(5) Has it occurred to you that she stopped talking in daycare at about the time you became pregnant with your younger daughter? Is she trying to be “more like a baby?” It’s possible. Furthermore, a 7-month-old baby is a time-intensive creature. You may have less time for her since the birth of the baby, and her behavior in preschool reflects her anxiety about her own position at home.
Well, my answer is longer than your question. I hope these suggestions help. However, if after several months, she still isn’t talking to other people, I would find a good clinic that offers behavior therapy and have her evaluated. Depending on their recommendations, you can arrange therapy for her.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.