Sex & Your Body

In order to understand how pregnancies occur in the first place, you'll need to learn a little about your menstrual cycle, hormones and ovulation (release of the egg). As you'll see, it's not very complicated.

What is a Menstrual Cycle?
Simply put, it's the preparation a woman's body makes for pregnancy every month. It's called a cycle because the same hormonal and physical events occur in sequence each time to prepare an egg for fertilization. The average length of a menstrual cycle is 28 days; however, it can be up to a week shorter or longer and still be normal.

Let's take a closer look at menstrual cycles: When a girl reaches about the age of 12, her body has already gone through many changes internally and externally. Menstrual cycles are a final step in puberty and signal the beginning of her ability to bear children. Her brain, ovaries and uterus have established a working relationship to produce an egg each month and a place for it to grow, if it is fertilized.

What Does this Machinery Look Like?
A uterus is located in the female pelvis and is roughly the size and shape of a small pear (2x6 cm). The lower portion opens into the top of the vagina (about a finger length inside) and is called the cervix. From the wider or upper portion of the uterus extend two delicate tubular structures known as Fallopian tubes. Located at the end of each tube is an ovary.

How Does It All Work?
The brain sends a chemical message through the blood stream to the ovaries. These "hormones" tell the ovaries to start readying an egg for ovulation. They also inform the ovaries to produce a powerful female hormone called estrogen. Estrogen has many effects throughout a woman's body, including stimulation of the growth of the inner lining of the uterus. This lining is called the endometrium.

About two weeks after the start of a woman's last period, an egg is released from one of her ovaries. This event is known as ovulation. The released egg is then picked up by one of the Fallopian tubes to await fertilization. Over the next several days, the ovary produces more estrogen and another hormone called progesterone. This substance puts the finishing touches on the uterine lining, making it ready for the arrival and growth of a fertilized egg.

What Happens if a Couple Has Sex Near the Time of Ovulation? Sperm from the guy's penis enters the vagina when he ejaculates (a little is also released just before!). To help his sperm travel, she secretes a watery mucus. The sperm find their way from the vagina through her cervix and uterus, and eventually travel out to the ends of her Fallopian tubes. There they search for an egg, sometimes waiting more than four days. If they find the egg, the sperm attempt to penetrate it. Fertilization is the successful joining of an egg and sperm, triggering the start of a pregnancy.

As soon as fertilization occurs, the genetic material within the two sex cells (egg and sperm) combines. Shortly thereafter the new embryo-to-be begins dividing: one cell becomes two cells. Two becomes four. Four becomes 16. (You get the point.) After three days, the microscopic cell mass travels down the Fallopian tube and burrows into the thick lining of the uterus. In most cases, approximately eight-and-a-half months later, a baby is born.

What happens when there's no fertilization by sperm? If fertilization doesn't occur within 1 to 2 days after the egg is released, the egg dies. As a result, estrogen and progesterone levels drop. Pregnancy won't happen during that cycle. This withdrawal of hormonal support causes the built-up uterine lining to shed. This is called a period, or menses, or menstrual flow! It usually lasts three to six days. At this point, the stage is now set for the entire cycle to repeat: the brain notices the low estrogen levels and starts to stimulate the ovaries again.

What's important to remember from this is pregnancy can happen at almost any time of the month! Ovulation isn't always predictable and the millions of sperm released are both patient and determined in the quest to find an egg. The only way a sexually active couple can reliably prevent pregnancy is careful, consistent use of a birth control method. Understanding how human fertilization works is a great first step as well. Remember that having a child is obviously a huge responsibility. It's also one of life's greatest experiences, but we all know it's best to wait until both of you are ready!

Be safe out there on the highways of love!

The HealthCentral Editorial Team
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The HealthCentral Editorial Team

HealthCentral's team of editors based in New York City and Arlington, VA, collaborates with patient advocates, medical professionals, and health journalists worldwide to bring you medically vetted information and personal stories from people living with chronic conditions to help you navigate the best path forward with your health—no matter your starting point.