What is the source of waiting three
months before announcing my pregnancy? Whom can I tell beforehand?

Shalom, Mazal Tov on your pregnancy-
and may you be blessed with a birth at an auspicious hour,
in health and blessings for you and your baby.
In connection with what you
asked about when and to whom you can tell you are pregnant.
I have searched far and wide,
and have not been able to find any definite sources on this
subject. I am not even sure that there is a real custom in
this area. It may rather be that the practice not to reveal
one's pregnancy is just a social norm, in order to avoid any
undue discomfort at having afterward to tell people if there
was a miscarriage (may we be blessed to all have healthy pregnancies).
If this is so, then one can
certainly feel free to tell anyone one wants to, at any date.
(I have been led to this belief after being informed that
even many non-Jews feel uncomfortable about revealing their
pregnancies at an early date. This would seem to indicate
that we are talking about a wide-spread social custom and
not a "custom" in the religious, legal sense of
the word.)
On the other hand, it may well
be that this is a real custom that exists amongst Jewish women-
not to reveal the pregnancy until three months have passed.
(I was led to this belief after being informed that the custom
amongst Chabad Lubavitch women is to wait five months until
sharing the news. This would seem to indicate that there is
a real custom that has defined practical boundaries). The
basis for the custom may be based on the words of our rabbis
in the Talmud, that a blessing only falls on things that are
kept secret or modest (ain habracha chal ele al davar hasmoi
min haayin). Or perhaps it is based on the concept of Ayin
Hara, the "Evil Eye." In fact, we find in halacha
(Jewish Law) a discussion of whether a women may pretend to
be menstruating, when in fact she is pregnant, in order to
avoid the "evil eye" of other (jealous) women, which
may cause her to miscarry.
However, even according to this
second understanding, it seems to me that the practice is
to inform some people. First of all- your husband! He certainly
will want to share your joy, help you both prepare (physically
and spiritually) for the birth. Also, you should inform your
medical care-taker, in order to assure that you get the correct
physical care and preparations you will need. Most people
also tell their parents (and parents in law.) And, of course,
you can share the news of the pregnancy with a rabbi in case
you need to ask him a pregnancy-related question.
Halachically, then, I have not
found any source to forbid telling everyone that your are
pregnant straight away. And if there is a need for this, one
could certainly reveal the news as required (e.g. if you are
an astronaut, and NASA is planning ot send you up in the space
shuttle in eight months time, it would be a good idea to tell
them to tailor you a suite with room for your obstetrician
to join you for the ride]. One the other hand, the practice
of modesty in this issue is praiseworthy, and apart from the
immediate family, it would be proper not to spread the news
too early (earlier than three months).
May you be blessed with only
good and a healthy and holy birth.
Rabbi Da'vid Sperling