Dr. Oz and Dr. Roizen on Pregnancy and Your Genes

On the other side of the equation, of course, we have those little swimming sperm. As with a woman's eggs, each sperm contains a single copy of each gene in the human genome.

Unlike women, men don't have a preset number of their reproductive players. In fact, a man produces more sperm in each ejaculation than the total number of eggs that a woman is endowed with for life. (evolutionarily, a man can continue reproducing for the majority of his adult life, maximizing the chance of passing on his genes.

A woman's reproductive life is limited to the younger years of her life because of the physical strain of pregnancy, childbirth, breast-feeding, and child rearing.)

A man's sperm, which is carried in semen that's made by glands such as the prostate, is stored in a duct called the vas deferens. When a man ejaculates, the sperm-carrying semen fires out through the urethra in a seek and-conquer mission. It may seem that all these millions of sperm are racing one another to the finish. But just like a tour de france cycling team, the sperm have different roles.

Some are deemed the leaders of the pack, trying to be the first to cross the line. Others are designed to assist, specifically by blocking other men's sperm from making it to the finish line. Competitive little game going on in there, eh? The goal of pregnancy, of course, is for a sperm to find an egg during a precise window of opportunity and fertilize it.

The Union

The purpose of an orgasm isn't solely to make you feel good or provide gossip fodder for the neighbors. The biological purpose is to better the odds that this union between sperm and egg takes place.

On the woman's side, the mucous membranes that line the vaginal walls release fluids during intercourse so that the penis can slide with just the right amount of friction. As intensity and sensations build, the woman's brain tells the vagina and nearby muscles to contract. That contraction brings the penis in deeper. Why does that matter? It increases the chance of his sperm getting closer to the target.

During an orgasm, the cervix, located at the top of the vagina, dips down like an anteater and sucks semen up into the cervix (the cervix is a passageway connecting the top of the vagina and bottom of the uterus). The sperm is trapped in the cervical mucus until the release of the egg, and a signal then lets the sperm start the competitive swim up into the uterus. While it's by no means necessary to have an orgasm to get pregnant, women who orgasm between one minute before and forty-five minutes after their partner's ejaculation have a higher tendency to retain sperm than those who don't have an orgasm.

CLICK HERE to read more from "YOU: Having a Baby" and to view all of the charts, diagrams and images from the chapter.

Excerpt: 'YOU: Making a Baby'

On the man's side, orgasm is required, because during orgasm fireworks in the brain cause involuntary contractions in lots of muscles in his body. Those contractions help him penetrate deeper and squeeze the prostate to eject sperm deep into the vagina.

Now, the actual fertilization process happens this way: After the egg drops from the ovary, it travels through the Fallopian tube, where there's about a twenty-fourhour window when it can be fertilized. Since sperm live for up to a week in the cervix (they die after a few minutes of hitting the air), it's not necessary for two people to have sex precisely when ovulation occurs, as many assume.

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