A successful pregnancy goes through several distinct stages. A man’s sperm must reach, penetrate, and fertilize a woman’s egg. The resulting zygote must divide and form a blastocyst. The blastocyst much reach the uterus and
A successful pregnancy goes through several distinct stages. A man’s sperm must reach, penetrate, and fertilize a woman’s egg. The resulting zygote must divide and form a blastocyst. The blastocyst much reach the uterus and implant in the endometrium. The implanted blastocyst continues its development into an embryo and then a fetus.
At any point in this process problems can occur that interfere with a successful pregnancy.
Fertilization is the union of the female gamete (egg) and the male gamete (spermatozoa). Whether it occurs naturally inside the female reproductive system or with the assistance of reproductive technologies outside of the human body, the product is a structure called a zygote.
When a woman is ovulating she releases one egg into her Fallopian tubes (or more in the case of fraternal twins). During this time, a woman’s cervical mucus will thin, in preparation for sperm to pass through more effectively.
Following spermatozoa ejaculation inside the vagina, special secretions help them to swim through the cervix towards the uterine tube where fertilization takes place within 24-72h.
The fertilized egg or zygote then begins to move toward the uterus, as the cells divide into the next stage, a blastocyst.
Problems with Fertilization
There are a wide range of problems that can prevent fertilization, but they all have the same outcome. Something is stopping sperm and egg from reaching each other.
Absence of Sperm/Egg: If a woman has an annovulatory cycle where she doesn’t release an egg, or if no sperm reaches the egg because of azoospermia or low sperm count, then fertilization will not occur.
Sperm’s Ability to Reach the Egg: Poor sperm motility or movement, or poor chemotaxis (sperms ability to navigate toward the egg) can cause problems. Similarly, if a woman’s cervical mucus does not thin enough during ovulation, it may not let sperm reach the egg.
Sperm Penetration: Poor morphology, abnormal sperm shape makes it harder to penetrate an egg. Sperm also relies on a chemical reaction, called an acrosome reaction, that helps it create a hole in the egg to pass through. Poor or incomplete acrosome reaction could also inhibit fertilization.
2. Blastocyst Development
Soon after fertilization, the embryo is created from a small group of cells that are constantly dividing inside of a c