The first step of IVF is retrieving eggs from the ovaries. The number of eggs that can be retrieved typically decreases with age - women under 35 average about 21 eggs per cycle, while women over 42 typically retrieve fewer than 10 eggs. There is significant variation between patients, with some retrieving many more or fewer eggs than average. This decline reflects the natural decrease in ovarian reserve that occurs with age.

This model uses findings from Maslow et al. (2020) which analyzed 1 or 2 cycles of planned oocyte cryopreservation and found that the number of eggs retrieved typically decreases with age.

Not all retrieved eggs are mature enough to be used in IVF. The proportion of immature eggs increases slightly with age, from about 28% in women under 35 to about 35% in women over 42. This means that older women not only retrieve fewer eggs, but a higher percentage of those eggs may be immature.

This model uses findings from Maslow et al. (2020) which analyzed 1 or 2 cycles of planned oocyte cryopreservation and found that the proportion of immature eggs increases slightly with age.

When eggs are frozen for future use, some may not survive the thawing process. Modern freezing techniques (vitrification) have greatly improved survival rates, with about 90% of eggs successfully surviving the thaw. This rate appears to be consistent regardless of the woman's age at the time of freezing.

This model uses findings from Torra-Massana et al. (2023) which analyzed 5,362 oocyte donation cycles and found that the survival rate of frozen eggs is about 90% using modern vitrification techniques.

Using ICSI (where sperm is directly injected into the egg), about 84% of mature eggs successfully fertilize. Interestingly, research shows this rate stays fairly consistent regardless of maternal age, suggesting that age-related fertility decline is primarily due to other factors like egg quality and quantity rather than fertilization ability.

This model uses findings from Hanson et al. (2020) which analyzed 8,308 IVF cycles and found that the fertilization rate with ICSI is about 84.4% regardless of the age of the mother.

After fertilization, embryos need to develop for 5 days to reach the blastocyst stage. Some embryos will stop developing (arrest) before day 5. This becomes more common with age - about 34% of embryos arrest in women under 35, increasing to over 60% in women over 42. This reflects the declining quality of eggs with age.

This model uses findings from McCoy et al. (2023) which analyzed over a thousand IVF cycles and found that the day 5 arrest rate increases with age.

During IVF, embryos are tested for chromosomal abnormalities (aneuploidy). Embryos with large chromosomal abnormalities very unlikely to implant and develop into a healthy pregnancy.

This model uses findings from Franasiak et al. (2014) which analyzed 15,169 trophectoderm biopsies from 3,301 IVF cycles, showing clear correlation between maternal age and aneuploidy rates. Aneuoploidy is one of the main reasons why IVF success rates decline with maternal age

Preimplantation genetic testing with whole genome sequencing (PGT-WGS) screening looks for single-gene disorders and other genetic issues beyond just chromosomal abnormalities.

This model uses analysis from Katz and Slotnick (2024) which compared Orchid's PGT-WGS screening panels to studies of rare monogenic disease in the overall population and estimated a roughly 3.7% rate of monogenic findings.

When screening embryos for specific genetic conditions (PGT-M), the proportion of embryos affected depends on the inheritance pattern and which parent(s) carry the variant. For autosomal conditions (chromosomes 1-22), the rates follow Mendelian inheritance. For X-linked conditions, the rates depend on both inheritance pattern and embryo sex.

For X-linked conditions, males and female embryos are affected differently because males have only one X chromosome. For example, when the sperm source has an X-linked dominant variant, all female embryos will inherit his affected X chromosome, while male embryos receive his Y chromosome and are unaffected. This leads to some situations where the sex ratio of the embryos after screening is not 50% male and 50% female.

Some parents who already have children choose to perform sex balancing to complete their family, only considering either male or female embryos for transfer. This is done to achieve a more balanced family composition.

Sex balancing reduces the number of available embryos by approximately 50%, as embryos of the non-selected sex are not considered for transfer. This can significantly impact the number of transfer attempts possible from a given number of embryo creation cycles.

Even chromosomally normal embryos don't always successfully implant in the uterus.

This model uses findings from Chang et al, 2016 which indicate that euploid embryo transfers show increasing failure rates with age, even for chromosomally normal embryos.

For embryos that successfully implant and have been screened as chromosomally normal (euploid), the risk of pregnancy loss is about 10%. This rate stays fairly consistent across age groups, suggesting that once a euploid embryo implants, maternal age has less impact on the chance of successful pregnancy.

This model uses results from Munné et al 2019 which found a roughly consistent 9.9% miscarriage rate on embryos determined to be euploid via PGT-A testing across all age cohorts.