include women who have a history of miscarriages, perimenopausal women, and those having in vitro fertilizations.
On the female side of the conception equation lie her eggs, which are fully formed and stowed away in her ovaries from before birth. Each mature egg contains one copy of each gene in the human genome—half the amount necessary for life. The maximum number of eggs that a woman will ever have is the number she has when she is a twenty-week-old fetus. She’ll have about 7 million of them then, 600,000 when she’s born, and about 400,000 at puberty. Once a woman hits puberty and menstruation begins, her ovaries release one of those eggs every twenty-eight or so days. During each cycle, even though multiple eggs start to develop, hormonal signals ensure that only a single egg will be released and the other eggs will regress. (It’s not wise evolutionarily to blow them all at once, so the body gives females an approximately thirty-year window in which to conceive.) Hormones also work to mature that ready-to-drop egg and to pop a hole in its sac. That hole works as an escape hatch, so the egg can slip out of the ovary and travel down the Fallopian tube, where it may be fertilized by sperm.* Tissue left behind in the ovary after the egg is released, called the corpus luteum, will produce hormones essential to successful pregnancy if the egg is fertilized.
The Sperm
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.
Print Shop
The word imprinting may sound like something you’ve heard on CSI, but it’s actually a form of epigenetics. Even though two copies of a given gene are inherited, one from mom and one from dad, in certain circumstances, one is permanently turned off. The nonexpressed copy is said to be imprinted. As of now, we know of at least eighty genes that are imprinted by epigenetic markers, causing them to be active or inactive in the offspring based on parent of origin. In general, expressed genes that are inherited from the mother conserve maternal resources and limit fetal growth, while expressed genes inherited from the father promote fetal growth, even if it means hurting the mother.
Problems can occur when genes that are supposed to be imprinted, or turned off, are not, or when the wrong parent’s gene is imprinted. The gene for the chemical messenger called insulinlike growth factor 2 (IGF2) is normally turned on from the father and off from the mother. If the mother’s copy is not turned off, the child can develop Wilms’ tumor, a cancer of the kidney. Loss of imprinting of the mother’s IGF2 gene later in life can contribute to age-related cancers, including cancers of the prostate and colon.
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
Figure 1.1 Tube Ride In the Fallopian tube, an egg has about twenty-four hours in which it may be fertilized. Once the sperm does so, the fragile combo, the blastocyst, multiplies its cells and must implant in the uterine wall to endure the 280-day pregnancy. Even if you try to summit Everest, this is the most dangerous journey you will ever take.
What’s Age Got to Do With It?
We all know plenty of people who have made the classic clock-ticking jokes about aging women who want kids. But what does that really mean? Before ovulation, eggs have two copies of each of the twenty-three chromosomes. They’re lined up waiting for the signal to divide for mom’s entire life. Unfortunately, the little spindles that pull chromosomes apart don’t work as well when they’ve been waiting for four decades. Instead of a clean break, two copies may be pulled to one side and none to the other. That’s what leads to an increased risk of chromosomal abnormalities such as Down syndrome and an increase in miscarriages in older moms.
Now, that doesn’t let pop totally off the hook. Older men’s (as in over 35) sperm have been linked to an increase in birth defects and autism, as well as an increased difficulty conceiving. New evidence even suggests that children born to older dads score lower on various brain tests through the age of seven.
While older parents may be better equipped to handle some aspects of pregnancy and child rearing (like some of the stresses and emotional wear and tear), and may be better able to support their children financially, there are some physiological trade-offs that you’ll want to consider if you are making a decision about when to have children.
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. 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-four-hour 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. In fact, conception is more likely to happen if sex occurs a couple days before the egg is released from the ovary. (See “Fertility Issues” on page 380 for more about getting the timing right.)
If all goes according to plan, the sperm meets the egg in the Fallopian tube, and the two half genomes unite to form a complete set of genes containing all the DNA necessary to make a new human being. The fertilized egg says thank you very much and moves along to the uterus. There it will attach to the uterine lining and begin the amazing process of becoming a baby.