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A handful of factors contribute to a successful pregnancy.

There's much more to the start of pregnancy than a sperm and an egg, says the American College of Obstetricians and Gynecologists—it takes two healthy reproductive systems, the right balance of numerous chemicals and the right timing.

Setting the stage

The sperm come from the man's testicles, two egg-shaped organs that rest in the scrotum, a pouch that hangs behind the penis. The testicles are suspended in this pouch to help keep them cooler than the rest of the body. At normal body temperature the testicles lose their ability to produce sperm.

The egg, or ovum, comes from one of the woman's two ovaries. These almond-shaped organs sit in the abdomen, about halfway between the navel and the vagina. The ovaries sit just beyond the fallopian tubes, two tubes that extend from each side of the uterus.

Every month, an egg matures and leaves an ovary. The egg enters one of the fallopian tubes and moves toward the uterus. Tiny hairs in the tube help propel the egg toward the uterus. The tube's walls also contract to push the egg along.

Meanwhile, the lining of the uterus becomes thick with tissue and blood. This thick lining will provide a home and nourishment if an egg is fertilized.

How they meet

During sexual arousal, the penis fills with blood and becomes erect. Muscles in the testicles contract, pushing sperm toward the penis. The sperm mix with fluid from the prostate gland and seminal vesicles. These fluids help move the semen into the penis, add bulk to the semen, help neutralize acids in the vagina that could kill the sperm and add sugars to the semen that nourish the sperm after they leave the body.

A man ejaculates when the muscles at the base of the penis contract, forcing the semen out the tip of the penis. The ejaculate may contain as many as 500 million sperm, notes the American Academy of Pediatrics.

In a woman, the vagina expands and beads of fluid form on the vaginal walls. This fluid eases penetration and reduces the acidity of the vagina, making it easier for sperm to survive.

Once the sperm enter the vagina, they swim toward the uterus by lashing their long tails. A fraction of the sperm make it into the uterus and continue traveling up the reproductive tract into the fallopian tubes.

When a sperm reaches the egg, it releases a chemical that breaks down the egg's outer coating. As soon as one sperm makes it into the egg, the egg's surface changes to lock out all the other sperm.

After they join

Once the sperm enters the egg, their chromosomes join. Chromosomes are long strands of genes. These genes encode human traits from eye color to foot shape.

The egg and sperm each carry 23 chromosomes, while each human cell normally has 46 chromosomes. Together the genes from the sperm and egg create a human cell with a 46-chromosome chain.

This fertilized egg, now called a zygote, travels toward the uterus. It implants in the thickened uterine lining, where it grows into an embryo and eventually a fetus.

Other factors

At minimum, a successful pregnancy requires:

• A healthy egg and healthy sperm.
• Unblocked fallopian tubes.
• The sperm being able to fertilize the egg.
• The fertilized egg implanting properly in the uterine lining.
• A healthy embryo.
• The correct balance of hormones in the woman's body to allow the embryo to develop.

Timing is also essential. An egg can only be fertilized for a couple of days after it leaves the ovary, while sperm can only fertilize an egg up to five days after they enter the vagina, says the American Pregnancy Association.

Despite its complexity, conception occurs in most couples within a single year of trying. Couples who aren't able to conceive after a year of trying may want to talk to a doctor, says the college. If you're over 35, talk to a doctor if you haven't conceived after six months.

Reviewed 3/30/2024