The Story of the Egg and the Sperm

By Jenny H. | Updated: Jun 18, 2020

Sexual intercourse can result in pregnancy

Everyone knows what sexual intercourse can lead to pregnancy, if the appropriate action is no taken to prevent it. So, it's a possibility that when you have unprotected sex, the sperm meets with the egg and a zygote is created. But what exactly happens? Why it is that pregnancy isn't the result of every episode of unprotected sex? And why do some women struggle to become pregnant? In this article we follow the sperm's journey and learn what part our sex hormones play in determining whether the women becomes pregnant or not.

A Sperm's Typical Journey


The act itself

Testosterone will have played its important role prior to sex, by increasing the woman's libido and sexual responsiveness. Estrogen will have also increased the moisture in the vagina once arousal had begun. This facilitates intercourse, making it more comfortable and enjoyable for both parties involved.


The sperm enters the vagina

When the man ejaculates or the sperm enters the vagina and, as long as there isn't a type of barrier contraception in the way, it will begin its journey up through the cervix and into the womb.


The sperm travels through the cervix to the uterus

The ease of this journey will depend on what stage of the menstrual cycle the woman is at. In the first 14 days of the cycle, the follicle stimulating hormone (FSH) aids the release and maturity of follicles from the ovaries. These follicles produce estrogen which changes the texture of the cervical mucus, so that it is thicker, stretcher and slipper, making it easier for the sperm to pass through.


The sperm travels from one side of the uterus to the fallopian tube

The sperm travels from one side of the uterus to the fallopian tube

High estrogen levels promote the production of another hormone called luteinizing hormone (LH), which encourages the release of the mature eggs in the follicles (ovulation). The abandoned follicle becomes a “corpus luteum” and this starts to produce progesterone after the egg has left. Most sperm won't make it to the fallopian tube, depending on the quality of the sperm.


The lucky sperm

Once the sperm reaches the fallopian tube, they must wait for a woman to ovulate. Sperm can typically survive five days, so if the woman doesn't ovulate within this time then the sperm die. The egg travelling down the fallopian tube will remain in a fertilizable state for about 24 hours. During this stage the progesterone that was produced by the corpus luteum maintains the lining of the uterus so that a fertilized egg can implant. Estrogen levels also increase again to thicken the endometrium, for when implantation occurs.


The egg is fertilized and a zygote is created

Once a sperm is attached to the egg, its tail is lost and the head becomes bigger. The two merge and create a zygote. This zygote moves towards the womb, which can take up to three days. It then hovers in the womb for between six and fourteen days, quickly multiplying cells and increasing in size.


 Implantation in the uterus

Implantation in the uterus

Once the zygote has developed sufficiently, it becomes known as a blastocyst, and attaches to the endometrium with help from estrogen. It is implanted into the thick lining of the uterus that has been prepared by the progesterone. When this happens, pregnancy has been achieved. The sperm has done its job. What develops after this stage is called the embryo, until eight weeks when it becomes a fetus.

More Information

This process sounds complex, but it is important to know about this journey to better understand how pregnancy occurs. For many couples, having sex at the right time is vital for achieving conception. To learn more about the role of hormones that dictate this intricate system, follow this link.

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