What causes abnormal fertilization?

A woman can only conceive a healthy fetus after normal fertilization, which is also a necessary condition. However, some women are found to have abnormal problems after fertilization. So what causes abnormal fertilization? This has a lot to do with the quality of sperm, such as low sperm motility, or dead sperm and few sperm, which will cause abnormalities. If you want to conceive a normal fetus, poor sperm quality is not good. You can get treatment first and then fertilization.

1. Evaluation of abnormal fertilization

16-18 hours after in vitro fertilization, observe the formation of pronuclei and the number of polar bodies in the fertilized eggs under a microscope. Abnormal fertilization is observed if there are no two pronuclei. Under an optical microscope, abnormal fertilization mainly occurs in the following forms: no PN, 1PN, and multiple PN.

2. Mechanism of abnormal fertilization

(I) No PN

During in vitro fertilization, approximately 30% of oocytes show no PN. Possible causes include:

1: Poor sperm quality, defects in sperm adhesion and penetration mechanisms, and lack of zona pellucida receptors lead to penetration failure.

2: The sperm quality is good, the oocyte cytoplasm is immature or the number of oocyte chromosomes is abnormal.

3: The male and female pronuclei may fuse prematurely, causing the pronucleus to disappear.

4: Late cleavage fertilized egg. This type of embryo has a high rate of chromosomal abnormalities.

(II) 1PN

The incidence of 1PN during in vitro fertilization is 3%-6%. The mechanism of occurrence may include:

1: 1PN with only female pronucleus, sperm abnormality leads to obstruction of male pronucleus formation.

2: For 1PN with only male pronucleus, ICSI injection destroys the spindle body undergoing meiosis, resulting in obstacles to the elimination of the second pole body, etc.

3: Parthenogenetic activation of oocytes.

4: The male and female pronuclei fuse to form one pronucleus. Generally, the single pronucleus of IVF is usually diploid.

5: The development of male and female pronuclei is not synchronous.

(III) Multiple PN

The fertilized egg contains 3 or more pronuclei, which is called polyspermy. The main reason is that two or more sperms pass through the zona pellucida and enter the oocyte during fertilization, which is also called polyspermy. Possible mechanisms include:

1: Multiple sperms enter the oocyte. Immature or overmature oocytes, high sperm concentration, zona pellucida abnormalities, high progesterone and high androgen levels in follicular fluid, and other factors can cause multiple sperms to enter the oocyte.

2: Sperm enters abnormal oocytes. Normal sperm enters polyploid oocytes, resulting in polyspermy.

3: The second polar body is retained in the oocyte and the pronuclear fragments.

For various abnormal fertilization situations, from a safety perspective, priority should be given to transplanting well-developed normally fertilized embryos. However, for patients with a small number of eggs and no normal embryos, choosing to transplant abnormally fertilized embryos still has certain clinical significance.