After my previous post where I just used the Gemara as a reference, I decided to read more of the interesting discussions found on that page of Niddah 31. Take a look at what I found:
Our Rabbis taught: During the first three months the embryo occupies the lowest chamber, during the middle ones it occupies the middle chamber and during the last months it occupies the uppermost chamber; and when its time to emerge arrives it turns over and then emerges, and this is the cause of the woman's pains.
What chambers are the Rabbis talking about? The embryo/fetus sits in the same uterus the entire time once it sets itself into the uterine wall in the first few days of pregnancy. And the flipping over of the fetus isn't what causes the women pregnancy pain, that's the contractions which cause the cervix to dilate. And that happens regardless, if the infant flips or not.
The pains of a female birth are more intense than those of a male birth. R. Eleazar further observed, 'What is the Scriptural proof for this? When I was made in secret, and curiously wrought in the lowest parts of the earth, it does not say 'dwelt' but 'curiously wrought'. Why are the pains of a female birth greater than those of a male birth? — The female emerges in the position she assumes during intercourse and the male emerges in the position he assumes during intercourse. The former, therefore, turns her face upwards while the latter need not turn his face.
Scriptural proof for a fiction? Neither gender gives the woman a more painful birth compared to the other. And when birth occurs there is no difference in the baby's position. In any normal birth the baby's face will first be turned to the side so to fit through the woman's hips and will then quickly turn upwards. This is true for both boys and girls.
Our Rabbis taught: During the first three months marital intercourse is injurious to the woman and it is also injurious to the child. During the middle ones it is injurious to the woman but beneficial for the child. During the last months it is beneficial for both the woman and the child, since on account of it the child becomes well-formed and of strong vitality.
Modern health professionals agree that sex is not dangerous at all during a normal pregnancy. In fact, if anything, it is only during the last few weeks that sex should be avoided because there may be chemicals in the semen which will stimulate contractions leading to a slightly early birth.
His father supplies the semen of the white substance out of which are formed the child's bones, sinews, nails, the brain in his head and the white in his eye; his mother supplies the semen of the red substance out of which is formed his skin, flesh, hair, blood and the black of his eye;
This is just complete nonsense. Both parents provide half the genetic material which is responsible for all the anatomical structures of the baby. It's also the mother who supplies all the "building materials" for the baby and through which everything is made.
R. Isaac citing R. Ammi further stated: A woman conceives only immediately before her menstrual period, for it is said, Behold I was brought forth in iniquity; but R. Johanan stated: A woman conceives only immediately after her ritual immersion, for it is said, And in cleansing did my mother conceive me.
The menstrual period is the beginning count of the menstrual cycle. Then, according to Halacha, the girl must wait five days before she checks if the bleeding stops and then waits another seven days until she dunks in the mikvah. After that, day 12, she can have sex until the next menstrual period. According to science, a woman can become pregnant only during ovulation which occurs, on a 28 day schedule, generally on days 11-14.
So can a woman become pregnant after immersion on day 12? Sure, that's smack in the middle of ovulation. R. Johanan is right. R. Isaac though is 100% wrong, proof text or no proof text.