Poor fertilization rate
6 Replies
Vick - April 12

I am 34 years old with unexplained IF. I am currently in the 2ww after a fresh ivf cycle. This is my second fresh cycle and I got pregnant with my son after a FET.
For this cycle I stimulated with follistim as last time and 12 eggs were retrieved. I was shocked to hear that only 2 of my eggs were fertilized when 10 out of 14 fertilized previously. Luckily I had an 8-cell with no fragmentation and a 7-cell with little fragmentation on day 3. Do you know why my fertilization rate might have been so low? I haven't talked to my RE about this yet. My husband's SA has always been normal. I know that of the 10 fertilized eggs I had last time, 4 resulted in multi-nucleated embryos. I was told that this was within the normal range but wonder if this indicates a problem with my eggs that has worsened in less than 2years.
Could triggering with hcg too early result in immature eggs that are unable to be fertilized? My largest follicles were 18.5, 18, 17 and 16mm on the day of the trigger. I seem to remember them being in the low 20's for my first cycle. I just want to be thinking ahead on how to improve my next cycle if this one doesn't work.
Thanks in advance for your help.


Dr Smith - April 13

Good detective work. You already figured out what went wrong. Triggering with hCG when the follicles are still too small results in immature eggs, poor fertilization, increased polyspermy and multinucleation. These events are all related to the maturation of the cytoplasm of the eggs, which was cut short by triggering too early. If this cycle doesn't work (and that's still an IF at this point), the stimulation should be extended next time until your follicles are larger. Hope things work out with the two embryos that were transferred.


Vick - April 13

Thanks very much for your reply. At least that gives us something to work with for next time if this cycle doesn't work.
What does polyspermy mean? And does multinucleation mean that more than one sperm has fertilized the egg at the exact same time? It seems that placing the eggs in a petri dish with millions of sperm would result in a higher rate of fertilization by more than one sperm b/c more sperm are present and able to eat through the cell wall, in synchronization, before it becomes inpenetrable. Is this the case?


Dr Smith - April 13

You've been doing your homework, or you really were paying attention in biology class ;)

Polyspermy is when more than one sperm penetrates the egg. Under normal circumstances, when the first sperm penetrates, the egg releases enzymes that harden the protein coat that surrounds the egg, thus preventing additional sperm from penetrating. However, if the egg is borderline mature, there may not be a sufficient amount of enzymes or the enzymes are released too slowly to prevent additional sperm from penetrating the protein coat. In IVF, we observe polyspermy in about 3% of the embyos. High levels of polyspermy indicate that some of the eggs were borderline mature (i.e. the cytoplasm had not matured sufficiently while the egg was in the follicle).

Multinucleation is different. When the embryo is preparing to divide, the DNA is duplicated. However, when cell division occurs, the DNA is not equally divided between the two daughter cells. One cell ends up with all of the DNA and the other gets none. The embryologist then sees more than one nucleus in one or more of the embryonic cells (blastomeres). This is documented as "multinucleation" of the embryo. Multinucleation is also associated with cytoplasmic immaturity.


ZMom2Bee - April 14

One thing about this concerns me. My clinic administers HCG when the two largest follicles are 18 mm. I have read that other clinics wait until they are 20-22 mm. I am wondering if my clinic's protocol will result in a smaller quantity of mature eggs?


Dr Smith - April 14

Although the criteria for the hCG trigger varies from one clinic to another, I'd say the consensus value for average follicle diameter is around 20mm. So, I'd have agree with you that triggering when the two largest follicles are only 18mm in diameter could result in an increase in the retrieval of eggs with immature cytoplasm.


Vick - April 14

Thanks for all the information. I will be well informed and ready for my next cycle if I need to be. I will push to wait for 20mm+ follicles next time!



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