Where Does Fertilization Occur?

For someone to become pregnant, the egg must be fertilized by sperm. After sexual intercourse, the egg and sperm join as one to create a zygote, or a fertilized egg. When this process is complete, the new zygote will start to grow into a fetus and ultimately become a baby. The process of fertilization requires several steps before it can be complete.

Where Does Fertilization Occur at?

For humans, fertilization primarily occurs in the Fallopian tubes. In actual practice, fertilization can take place in the top part of the uterus, outside of the reproductive tract or in the Fallopian tubes. If there is an inflammatory disease, the egg could even become fertilized in other parts of the pelvis. When this occurs, it leads to an ectopic pregnancy that can be life threatening if it is not treated.

The only time that fertilization can occur is during ovulation. At this time in the menstrual cycle a group of eggs begin to grow within the uterus. They grow in small sacs known as follicles and will continue to grow until one egg is released from the follicle. When this happens, the woman is ovulating and can get pregnant. Normally, ovulation takes place about two weeks after the first day of the woman’s period. Depending on her body chemistry, normal period length and other factors, the woman may actually ovulate sooner or later than this date.

How Does Fertilization Take Place?

The entire process of fertilization centers around the woman’s menstrual cycle. The menstrual cycle is designed to prepare the woman’s body for pregnancy and fertilization every month. If this does not occur, then her period will start like normal and the body will restart the entire process. Throughout the process, the egg is released, travels down the Fallopian tubes and prepares for fertilization.

During sexual intercourse, the man ejaculates semen into the vagina. At this moment, as many as 150 million sperm flood the body as they prepare for fertilization. Ideally, the sperm will reach the egg in the Fallopian tube. They can only live for 12 to 48 hours in the body, so the sperm must travel fast to reach the egg. Approximately 85 percent of sperm will be able to reach the Fallopian tubes, and only 15 percent of sperm actually make it to the egg.

To reach the egg, the sperm follow signals given by the cervix and the vagina. They travel up the cervix and into the lining of the uterus. About 1,000 sperm reach this point and prepare to enter the Fallopian tubes. At this point, there are two Fallopian tubes to choose from, and the sperm must make the right choice if they want to reach the egg. If the sperm chooses the right tube, they will have a chance of fertilizing the egg. If the egg has already been released during ovulation, the sperm can find the egg within just 20 minutes. Out of the 150 million sperm that started, a mere dozen or so will reach the egg, start circling it and try to fertilize the egg.

How the Process of Conception Works

It can be almost impossible to pinpoint the exact day that you became pregnant on. While your doctor will estimate the date by counting forward from your last period, there is actually some variation in the date that you conceive.

Conception happens after the egg is released from the follicle. This happens about 10 to 14 days before your next period is supposed to start. Once the egg is released from the follicle, it transforms into the corpus luteum. This causes the body to release extra hormones in preparation for a pregnancy. The lining of the uterus becomes thicker in case fertilization occurs.

From this point, the egg moves to the Fallopian tubes. It will stay in the Fallopian tubes for about 24 hours while it waits for a sperm to fertilize it. If the egg is not fertilized, it will continue its path through the uterus and disintegrate. Afterward, hormones will fall to normal level and the uterus will shed its lining for your period to start.

When the Egg Becomes Fertilized

If the egg is fertilized, it does not matter if any more sperm are present—none of them will be able to penetrate the egg any longer. The moment that the egg is fertilized, the gender and genetics of the baby are set. If the sperm had an X chromosome, the baby will be a girl. If the sperm had a Y chromosome, the baby will become a girl.

For the next few hours or up to four days, the egg will remain within the Fallopian tubes as the cells start to divide. Once this is done, the egg will move to the uterus where it will implant itself on the side. Three weeks after implantation, the nerve cells of the baby will begin to form.

After implantation, the woman may experience cramping as her uterus makes room for the baby. Hormone levels of hCG will begin to rise. Since it can take up to four weeks for hCG levels to increase, a pregnancy test may not appear positive quite yet. Once hCG levels are high enough, the hormone will be detectable on test strips.

Things to Remember

Ovulation Is Not Always Clear: It is not always possible to tell when you are ovulating. Normally, ovulation occurs at the same time as vaginal secretion turns white and sticky like egg whites. Ovulation test kits can also help to determine when you have ovulated, or you can measure your basal body temperature.

One in a Million: Fertilization is pretty amazing. After 150 million sperm are released during ejaculation, the sperm have just 12 to 48 hours to reach the egg. Ultimately, only one out of the 150 million sperm will actually implant the egg.

Implantation Happens in the Uterus: Even when the egg has been fertilized, implantation will not occur until it reaches the uterus. This can take up to four days to happen.

Implantation Bleeding: As the egg nestles into the uterine wall, it can cause some shedding and bleeding. This bleeding will be extremely light. If it is heavier or lasts for several days, it may mean that your period started or you had an early miscarriage.