Understanding ovarian reserve: will I really run out of eggs?

What is ‘ovarian reserve’?

‘Ovarian reserve’ refers to the pool of eggs left in your ovaries with reproductive potential. Because women are unable to make new eggs, your egg supply (and fertility) naturally declines with age until menopause when you no longer ovulate at all.

A bit of biology

Your ovaries are oval shaped, around 3–5 cm long and found on either side of your uterus (womb). Ovaries contain follicles which act like a nest for immature eggs. Each month, your hormones stimulate many of these follicles, triggering the eggs inside them to mature. Even though a number of follicles are activated (often referred to as the ‘cohort’), the ovary will usually only release one dominant egg each month (‘ovulation’), leaving the other eggs to deteriorate. If the mature egg released from the ovary meets and unites with any sperm present in your reproductive tract (‘fertilisation’), pregnancy results (as long as the fertilised egg continues to develop and successfully implants in the wall of the uterus). This is why sexual intercourse is recommended around the time of ovulation.

Egg numbers change as we age

Even though women start with 1–2 million eggs, by the time you reach puberty only about 300,000–400,000 of the eggs you were born with remain. The monthly cycle described above then continues throughout a woman’s life until there are no eggs left. You will have around 27,000 eggs remaining in your late 30s and around 1000 at the onset of menopause. As the total number of remaining eggs decreases, so does the size of the monthly ‘cohort’. This means there are fewer follicles from which the ovary is able to select an egg for release each month and fewer potential eggs available for pick-up when an older woman undergoes egg collection for IVF or egg freezing. In total, your ovaries will release around 500 mature eggs throughout your fertile years (puberty to menopause).

It’s not hard to see why age plays such a major role in a woman’s potential to fall pregnant. Generally, the most fertile period of a woman’s life is between her 20s and early 30s – when egg quantity and quality are at their best. Women will typically find it more difficult to fall pregnant after the age of 35, when there is a sudden, sharp decline in egg count, along with a reduction in egg quality.

Counting eggs

There are two tests commonly used by fertility specialists to check a woman’s ovarian reserve. The first is the AMH (anti-Mullerian hormone) test, more colloquially known as the ‘Egg-timer test’. AMH is a hormone secreted by the follicles in the ovaries. However, as a woman matures, AMH levels naturally decrease. By measuring AMH levels in the blood, we can gauge how many eggs remain in your ovaries – high AMH levels are a strong indicator of a high number of eggs.

The second test we use is called an ‘antral follicle count’. This test involves viewing your ovaries via an ultrasound scan (typically in the first week after a period). This enables us to count the number of egg-containing follicles in your ovaries.

It’s important to note that although both these tests provide valuable information about egg quantity, neither is able to assess egg quality.

Can I improve my ovarian reserve?

While there aren’t any proven ways to hold onto your eggs for longer, certain lifestyle factors have been shown to accelerate egg loss. For example, cigarette smoking may lead to the premature deterioration of your eggs. Thus, avoiding smoking is one of the simplest ways to preserve your fertility.

Are there other reasons my ovarian reserve may be low?

While a low (or diminished) ovarian reserve is a normal part of every woman’s biological clock, egg loss can occur earlier than expected for some women. Besides smoking, common causes of diminished ovarian reserve include various genetic conditions (e.g. Fragile X), ovarian surgery (e.g. for endometriosis or ovarian cysts) and some cancer treatments (e.g. certain chemotherapies and radiation therapy).

What role does egg freezing play in fertility preservation?

It is now very common for women who expect to start a family later in life (when they know their natural reserve of eggs will be lower) to consider freezing some of their eggs. Freezing your eggs is a safe and effective form of fertility preservation that can help limit the impact of expected age-related fertility decline. The best time to freeze your eggs is in your 20s and early 30s (when you are most fertile and you have a lot of good-quality eggs).

Egg freezing involves four steps:

• Ovarian stimulation: We stimulate the ovaries with hormone injections to promote the development of egg-containing ovarian follicles
• Monitoring: We monitor the developing follicles via ultrasound to help guide the timing of egg collection
• Egg collection: Under light anaesthesia, we collect around 15–20 eggs (but this number is highly dependent on the ‘cohort’ size)
• Storage: We identify and freeze any mature eggs, which can then be stored for up to 10 years.

If a woman encounters difficulty falling pregnant when she is ready to start a family, she then has the option of thawing her frozen eggs and trying for a baby via IVF.

Falling pregnant when egg numbers are already low

Women with a low ovarian reserve may still be able to have children with the help of assisted reproductive techniques like IVF. However, IVF success is reduced in women with low ovarian reserve and the risk of miscarriage is higher due to lower egg quality. If a woman’s ‘cohort’ size (i.e. the number of ovarian follicles activated each month) is significantly reduced, even very high doses of stimulating hormone will only produce one or two poor-quality eggs for collection during the IVF cycle. In this case, using donor eggs instead may be an option for some women.

Discussing your options

If you would like to learn more about low ovarian reserve or understand if your egg supply could be affecting your fertility, you can make an appointment with one of our fertility specialists by calling (03) 8080 8933 or by booking online.