Eggs and age is a issue most women underestimate until they want to start a family, and this is mainly because they weren’t told about it, in fact many women underestimate the impact of age on their egg numbers by 10 years. Eggs are kept in a safe immature state, wrapped up in dormant ‘primordial follicles’ away from the rest of the world until they’re activated by Follicle Stimulating hormone (FSH) in a process called ‘recruitment’ that starts at puberty, and once this happens the little eggs grow and mature within an increasingly large and complex follicle, with one or two eggs eventually being released each month at ovulation.
Egg numbers are a really important indication of a woman’s fertility, and at puberty most girls have about 180,000 eggs in their ovaries, and the number falls to about 1,000 at the menopause, and the number of eggs in the ovaries is known as the “ovarian reserve“. There’s a hormone called the Anti-Mullerian hormone (AMH) that’s released by medium sized follicles as they develop during the recruitment process, and the levels of AMH can be used to:
- Calculate the current ovarian reserve
- Predict a woman’s response to drug stimulation
- Predict the number of eggs that will be collected in IVF
- Predict the age at menopauseiii
- However AMH levels can’t accurately predict embryo quality or the likelihood of a pregnancy!
A low ovarian reserve is a significant part of many couple’s fertility in the 21st century and it’s associated with:iv
- Being older
- Having a small ovarian reserve at puberty
- Accelerated ovarian ageing
- Damage to the ovaries
There are models that map changing egg/follicle numbers, and most women experience a gradual reduction in numbers up to about the age of 32-35. After this the fall in eggs numbers increases, which creates the ‘broken-stick’ pattern of egg/follicle loss seen in the graph below, with two distinct rates in the different age groups.
Differences in ovarian reserve have a major effect on monthly conception rates, and as there can be big variations in ovarian reserve at puberty (as well as egg loss rates), this means the ages of 32, 35 or 40 aren’t ‘magic numbers’ but are ‘markers’ of relative fertility for most women. Having fewer eggs/follicles in the ovaries affects fertility directly and indirectly, as when the ovarian reserve is low fewer follicles and eggs reach the last stages of recruitment. This means less estrogen production from the ovaries, which means less growth of the womb lining and possibly receptiveness at implantation time.
For a healthy pregnancy the most important issue is the quality of the eggs and sperm involved, rather than the issue of egg quantity as a healthy baby comes from a single healthy egg. This crucial issue is often overlooked as it’s impossible to test egg quality, and because more follicles and better egg quality is generally found in younger (more fertile) women, the numbers of eggs/follicles at the last stages of recruitment are used to indicate egg quality, but this isn’t necessarily the case.
The quality of the follicle and the egg that grew in it depends on how well they negotiated folliculogenesis (eggs and follicles), and many eggs get damaged or die during this long and complicated process, which reduces their quality, as well as the chances of the follicle developing into a healthy corpus luteum. There are a number of factors that we know affect egg and sperm quality, (as lifestyle explains) and the fall in egg quality with age is thought to be a reflection of:
i ‘Anti-Mullerian Hormone and Inhibin B in the Definition of Ovarian Aging and the Menopause Transition’ MaryFran R. Sowers et al., J Clin Endocrinol Metab. 93: 3478–3483, 2008)
ii ‘Anti-Mullerian Hormone and Inhibin B in the Definition of Ovarian Aging and the Menopause Transition’ MaryFran R. Sowers et al., J Clin Endocrinol Metab. 93: 3478–3483, 2008)
iii ‘The value of anti-Müllerian hormone measurement in the long GnRH agonist protocol: association with ovarian response and gonadotrophin-dose adjustments’ E. Anckaert et al. Hum. Reprod. (2012) 27 (6): 1829-1839.
iv ‘Subfertility reflects accelerated ovarian ageing’. Helen S. Kok et al, Human Reproduction, Volume 18, Issue 3. Pp. 644-648.
v ‘Total anti-oxidant status: a biochemical predictor of human male fertility’ A. L. Adeel et al. Andrologia Volume 44, Issue Supplement s1, pages 20–25, May 2012. DOI: 10.1111/j.1439-0272.2010.01131
vi ” Are apoptosis and oxidative stress increased in cumulus and granulose cells from oocytes of young women with low ovarian reserve?” Calonge, Rocio Nuñez et al. Reproductive BioMedicine Online, Volume 32, S1
vii “Protective effect of resveratrol against oxidative damage to ovarian reserve in female Sprague–Dawley rats” Pinar Ozcan et al. Reproductive BioMedicine Online Volume 31, Issue 3, September 2015, Pages 404-410