You can run a modified natural cycle IVF. That would mean retrieving one egg and hoping for ongoing pregnancy. Success rates vary, but are likely between 4% and 15% per cycle. This is the best case scenario for women with good-quality eggs. If the same individual were to add in medication, she may have success rates as high as 70%. Why so much higher?
When we retrieve eggs, a lot of them may look good under the microscope, but in fact not be functional. Not every IVF patient is the same but on average it takes four to five eggs to generate one blastocyst.
Blastocysts are the best developed embryos we have in the laboratory and will take five to six days to generate.
Blastocysts are associated with the highest pregnancy rates. Not every blastocyst is genetically balanced.
A blastocyst with the correct number of chromosomes is called “euploid”. For women that are thirty-five, on average 70% of blastocysts will be euploid; for women thirty-six to thirty-eight, it is closer to 50%; and over thirty-eight years, it is closer to 30%. By the time women are over forty-two, it is likely an even lower fraction.
Even euploid embryos may have smaller genetic or other irregularities such that it will be an approximately 80% implantation rate.
Looking at all the numbers above, then, for the average thirty-seven year-old patient, mathematically at least, we can expect pregnancy if there are three blastocysts to be transferred over one to two cycles. To generate three blastocysts, we would like to have generated fifteen eggs.
That is why our goal for IVF is to generate twelve to twenty eggs.
I have seen cases where there were only two eggs, leading to two blastocysts and an ongoing twin pregnancy. If you have a very low ovarian reserve, but every reason to believe you have extraordinarily high-quality eggs, it is reasonable to hope that IVF could work for you.
But for most women, if there are fewer than twelve to twenty eggs being generated through IVF, then there is a lower chance for success based on the math described above.
So why not more than twenty eggs? The answer is that over twenty-two eggs, there may be enough hormonal imbalances that pregnancy rates may start to fall again. With such high numbers, the current community standard is shifting to freezing all embryos and not completing the transfer at all, instead waiting for a future cycle when your hormones are likely to be in better balance.