Tazria

Parshat Tazria begins with the statement:
דַּבֵּר אֶל-בְּנֵי יִשְׂרָאֵל, לֵאמֹר, אִשָּׁה כִּי תַזְרִיעַ, וְיָלְדָה זָכָר--וְטָמְאָה שִׁבְעַת יָמִים, כִּימֵי נִדַּת דְּו‍ֹתָהּ תִּטְמָא.
"Speak unto the children of Israel, saying: If a woman be delivered, and bear a man-child, then she shall be unclean seven days; as in the days of the impurity of her sickness shall she be unclean."

Hyper-Literally, אִשָּׁה כִּי תַזְרִיעַ can mean "when a woman gives forth seed." The following is based loosely on a statement of Chazal.
We can make the following drasha:

When a woman gives forth seed first, before the man, then וְיָלְדָה זָכָר, she will give birth to a boy.

That is, if she ovulates first, the likelihood is increased that she will give birth to a boy.

Consider the info from the following web site:
http://www.bloodays.com/bloodays_online_help/sex_plan.html

This method is based on simple physiological facts:

"Male" spermatozoon more mobile, but less stable
"Famale" - less mobile, but more sturdy
Spermatozoon can survive in the uterus up to 4 days
Conception can happend during ovum pass down the
fallopian tube (i.e. during 3-4 days after ovulation)
so we made supposition based on previes facts:

If coitus happend before ovulation, then "female"
spermatozoon stand a better chance to live up to
fertilization
if coitus happend after ovulation, then "male"
spermatozoon stand a better chance to came to the egg first

and also here, based on Dr. Shettle's sex selection theory.

However, Wilcox published an article in 1995 based on a study that found no correlation between time of conception and the sex of the baby.

Still, see this opinion peice by James from 2000:
"There are five non-human mammalian species for which it has been reported that the time of insemination within the cycle is directly associated with the sex of the resulting offspring, i.e. white-tailed deer, Barbary macaque, golden hamster and Norway rat (for references, see James, 1996) and mouse (Tarín et al., 1999).
With regard to human beings, I conclude from previous data that there is a regression of sex ratio on cycle day of insemination such that boys are conceived earlier than girls (James, 1971). More recently, Guerrero (1974) and Harlap (1979) elaborated on this, offering evidence that the regression is U-shaped (rather than monotonic declining), and a meta-analysis concurred (Gray, 1991)."

If so, it makes a nice drasha. (Chazal I believe take tazria to mean something other than ovulation in their drasha.)

Also, while we are at it, let us see those 5 animals:

Barbary Macaque (Macaca sylvanus)


Golden hamster - MESOCRICETUS AURATUS


Norway rat - Rattus norvegicus


Mouse:



White Tailed Deer: