Certainly, any number of contemporary singers have children (Sills,
Horne and many more), and it would be interesting to know if there is
a real statistical difference between the number of women in opera -
assuming it's a profession <g> - and in other arts or that require
that level of education and preparation in whether or not they have
children.
I have the impression - purely that - that this was much less
something women did at various points in the past, although among the
most notable exceptions are Ritter (not Thelma) who was, among other
things, the first Virginie in Paul et Virgine by Masse, and her
daughter, Ritter-Campi, who has left us some recordings, and who
debuted in her mother's role.
I can't remember if any of the so called iconic singers of the 19th
century - Malibran, Sontag, and so on, had children, although for some
reason I thought Viardot Garcia did. Patti, obviously not.
It may also be in some cases that they had children and gave them up,
which is not something you can discount historically.
I have been told that menopause can play hell with a singer's support
and so on, and I also understand that after childbirth you really have
to take it easy in terms of getting the muscles back into shape.
I don't think this is just a women's issue, however and can't think of
a single castrato who ever fathered a child.