12 Mar An Overview of the 5 Steps of Conception from US Fertility Network
How Conception Works
To become pregnant, five steps must occur. These are sperm transport, egg transport, fertilization, embryo development, and embryo implantation.
The sperm must be deposited and moved to the site of fertilization, which is called sperm transport. Several factors are involved with the transportation of sperm. These are:
- Sperm must be capable of propelling themselves through the female reproductive tract and environment.
- The environment must be of favorable cycle hormonal control so the sperm will not be destroyed.
- The sperm should possess the ability to penetrate the outer cell membrane of the egg, which is called capacitation.
After ejaculation, the semen forms a gel that protects it from the vaginal acidic environment. The gel turns to liquid in 20-30 minutes due to prostate gland enzymes. For sperm transportation to occur, this liquefaction is important. The seminal plasma remains in the vagina. The sperm with the greatest motility (ability to move) will penetrate the cervical mucus, which lies at the base of the uterus.
During ovulation, the cervical mucus barrier thins and becomes less acidic, which makes a better environment for sperm. After entering the uterus, contractions will propel the sperm upward into the fallopian tubes. Sperm will enter the tubes only minutes after ejaculation, and motile sperm can survive in the female reproductive tract for 5-6 days.
When ovulation occurs, the egg is released into the fallopian tube, which is known as egg transport. The process of egg transport begins with ovulation and ends when the egg reaches the uterus. After ovulation, the finger-like ends (called fimbriae) of the fallopian tube sweeps over the ovary. On the fimbriae surface are tiny adhesive sites called cilia. These projections are responsible for egg pickup and movement into the fallopian tube.
The tiny cilia in the tube, along with muscular contractions, move the egg in a forward motion. The total time for egg transport is around 30 hours. Certain medical conditions can impair fallopian tube function, such as endometriosis and infection. These conditions damage the fimbriae and cilia, and cause serious scarring.
The union between egg and sperm is known as fertilization. After ovulation, the egg is capable of fertilization for around 12-24 hours. Contact between the sperm and egg is random. The egg makes it down to the ampullar-isthmic junction, which is a portion of the fallopian tube. It sits here for around 30 hours, awaiting fertilization. The fertilized egg transcends rapidly to the uterus.
Any defects in the fallopian tube could impair the movement of the fertilized egg, which increases the risk of ectopic (tubal) pregnancy. The membrane surrounding the egg is called the zona pellucida (shell). This structure has sperm receptors that are specific for human sperm. After the sperm penetrates this membrane, the zona pellucida will not allow other sperm inside.
After fertilization, an embryo develops. After initial fertilization, the single-cell embryo is called a zygote. Cell division begins, and over the next 7 days, the embryo undergoes mitosis, or multiple cell division. At the end of the transition period, the embryo develops into a mass of organized cells, which is called a blastocyst. As a woman ages, embryo development is impaired because of diminished egg quality.
During the implantation stage, the embryo implants into the lining of the uterus, which is called the endometrium. The embryo makes its way to the uterus, and 5-6 days following fertilization, it will hatch from the zona pellucida. Around 50% of all fertilized eggs are lost before the onset of menses (menstrual period). If the blastocyst makes it to the uterus, hatches from the shell, and implants, pregnancy occurs. The lining of the endometrium is thickened from hormone changes, which allows implantation to occur and the pregnancy to sustain.
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