It wouldn't be her property - it would be held in trust for the child.
If the child dies before he reaches majority the property reverts to the estate.
That's precisely the way my will works, for example.
If I die, my assets will be held in trust for my children until they reach age 35, with disbursements before that age for such things as medical care, education costs and other necessaries of life.
This is a standard way of protecting one's children in the event that one's spouse remarries after one's death to an unscrupulous individual. And, of course, if one suspects that one's spouse would get an abortion if she were pregnant when one died, there are ways of making that cost her dearly as well.
After all, a born child's property upon death passes to the parent. If an unborn child was considered a person of equal standing to a born child, an unborn child's property would also pass to the parent.
This is of cause in wills or trusts without specifics on the age of the beneficiaries (to keep the principles clear).