Basic Types of Sexual  Behavior

2. Heterosexual Intercourse: Genital Intercourse

Coitus

Better chance of pregnancy This coital position increases the chance of pregnancy because it allows for an ejaculation close to the cervix.

Techniques of Coitus 8
There are also women who prefer to remain rather passive themselves, and who enjoy lying on their backs while carrying the weight of a man's body. For them, the "missionary position" (man on top of woman, facing her) may indeed be the most rewarding. Furthermore, if they flex their legs in this position they make it possible for the penis to penetrate very deeply into the vagina, thus increasing the chance of a pregnancy. (The ejaculated semen forms a so-called seminal pool close to the cervix, and a woman can favor fertilization simply by remaining on her back for a while after coitus.) Furthermore, the deep penetration itself may be sexually rewarding for many women, and thus they may spontaneously shift to coital positions which make this possible.
Finally, it remains to be stated that people who are obese or weak usually are most comfortable during coitus while lying on their sides. Indeed, a rear entry approach in this position may be the least stressful of all. It naturally also recommends itself to women in the later stages of pregnancy.