The Biology of Making a Baby

Fertility in a Woman’s Body

For a woman, fertility begins in the ovaries, the two small, oval organs attached to either side of your uterus. Every baby girl is born with between one and two million eggs already in her ovaries. Many of these eggs begin dying off immediately, and the rest decrease in number as you age. Between her first period and menopause, the average woman releases 400 eggs.

At the beginning of your menstrual cycle, between one and three follicles begin maturing inside of your ovaries. Each follicle contains one egg. The most mature egg is released into one of your two fallopian tubes during the process of ovulation.

The egg lives for 12-24 hours after ovulation. It needs to be fertilized during this time for a baby to be conceived. If your egg meets a healthy sperm during its journey to the uterus, a pregnancy will begin. If not, the egg ends its journey at the uterus and disintegrates.

If you haven’t conceived, then about 14 days after ovulation you will begin your next period. If no pregnancy has occurred, your ovaries will stop releasing estrogen and progesterone, and levels of these hormones will drop. Your uterine lining and the remains of the unfertilized egg will shed during your period.

Cycle length and ovulation time varies from woman to woman. Most of the time, that variation happens before ovulation, during what is called the follicular phase of your cycle. The luteal phase—the time between ovulation and your next period—is almost always 14 days.

Fertility in a man’s body

Women’s bodies mature only one egg each month. Men’s bodies, in contrast, are constantly producing millions of tiny sperm. Sperm have a single mission: to penetrate a woman’s egg.

Each sperm cell takes two to three months to mature, and the average sperm cell lives only a few weeks inside a man’s body. Because roughly 40 million sperm are released every time a man ejaculates, his body needs to be constantly producing sperm.

Sperm production begins in the testicles, the two glands contained in the scrotum. There’s a reason why the scrotum hangs down below the body: healthy sperm require a cool temperature of 93 degrees Fahrenheit—four degrees cooler than normal body temperature.

Once the sperm is created, its stored inside each testicle in the coiled tube of the epididymis. Just before ejaculation, the sperm mixes with semen and is released.

What happens during sex

When a man has an orgasm during sex, he propels sperm-rich semen into the vagina and up toward the cervix at around 10 miles per hour. The force of ejaculation gives the sperm a running start on their journey towards the egg. (For a woman, an orgasm is certainly nice, but it’s not necessary for conception to occur.)

Does sex position matter? No one really knows. The most important thing is to make sure you’re having sex often during your fertile window. In order to conceive, live sperm needs to be in your reproductive tract during ovulation. Sperm can stay alive for several days inside your body when fertile cervical mucus is present.

What happens after sex

After ejaculation, millions of sperm have begun their journey to your egg. If sex occurs outside of your fertile window, one of the first obstacles that sperm will encounter is your cervical mucus, which is sticky and impenetrable. When you’re most fertile, cervical mucus loosens and promotes sperm mobility. It can even keep sperm alive for up to seven days, meaning that the sperm from sex you had several days before ovulation could still get you pregnant.

The sperm enters your cervix and swims through the uterus to the fallopian tubes. It’s a journey that can take anywhere from 45 minutes to 12 hours, depending on how fast the sperm swims. Most of the sperm will never complete this journey. They’ll head up the wrong fallopian tube, get lost or trapped somewhere, or die along the way. The few dozen that manage to reach the egg will try frantically to penetrate its outer shell.

Fertilization

If one sperm manages to enter, the egg changes instantly to prevent any other sperm from entering. Next, fertilization begins: the genetic material of the sperm and egg combine to create a new cell that rapidly starts dividing. This tiny bundle of cells is called a blastocyst, and it travels down the fallopian tube on a three-day long journey to the uterus.

You aren’t officially pregnant until the blastocyst attaches itself to the wall of the uterus, where it develops into an embryo and placenta. Two weeks later, when you miss your expected next period, it’s time to take a pregnancy test to confirm it.

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