Fertilization and Pregnancy

A man and woman in bed together

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Fertilization is the first step that needs to happen to make a baby. In simple terms, fertilization occurs when the sperm and egg combine to create an embryo. In order for a man and woman to make a baby, you need a brain, a uterus, fallopian tubes, sperm, and healthy eggs.

The Role of Follicles

In the ovaries, there is something called a follicle. The follicle is a fluid-filled sac that contains one egg and the cells that prepare the egg for early embryo development. When a woman has her period, there are a whole group of follicles in the ovary that is preparing to grow. These follicles, known as antral follicles are between 2 and 10mm in size.

The brain makes a hormone called follicle-stimulating hormone (FSH). FSH tells the follicles in the ovary to grow and get bigger. One gets to be about 20mm and all of the others disintegrate. That big follicle makes estrogen which tells the lining of the uterus to grow and get ready for a pregnancy. The brain responds with a hormone called luteinizing hormone (LH)—this is what’s typically detected by an ovulation predictor kit. LH is only present in the body for one day every month.

Ovulation and the Release of the Egg

Approximately 24-36 hours after LH appears, ovulation occurs and the egg is released from the ovary. The egg needs to be captured by the fallopian tube where sperm can find it. A fertilized egg is called an embryo. Five days after fertilization, the embryos at the blastocyst stage is pushed into the uterus, where it needs to implant into the lining. If this occurs, pregnancy hormone is made and the next period is missed.

The Process of Fertilization

Fertilization typically occurs in the fallopian tubes of a woman shortly after ovulation if sperm has made it into the reproductive tract. During intercourse, millions of sperm travel through a woman's reproductive tract but only one single sperm can penetrate or fertilize, the egg. Instantly, after the egg and sperm meet, cells start to mix together and divide which starts to form the genetic DNA sequencing and gender (boy or girl!) for the future baby.

If a single sperm is unable to penetrate the egg, that egg moves through the uterus and disintegrates. After that, your hormone levels will return to normal and a woman's body begins to shed the thick lining of the uterus through menstruation.

Once the egg is fertilized, it then moves down the tube and implants into the uterus, initiating a pregnancy. Some women may notice spotting (or slight bleeding) for 1 or 2 days around the time of implantation. Occasionally the fertilized egg will implant outside of the uterus, such as in the fallopian tube or cervix, which is known as an ectopic pregnancy.

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