Age and fertility

According to the British Fertility Society, over half of babies born in the UK are to women over 30 [1]. With so many opportunities to travel, relocate, or progress at work, it’s hardly surprising that more people are planning to have a family later than before.

But with one of the biggest determinants of female fertility being age — just how long can you wait?

Whether you are a persistent planner, or have just begun thinking about fertility, we look at how and why fertility changes with age, and the role of anti-Müllerian hormone (AMH) as an indicator of ovarian reserve. 

What age does fertility decline?
How does age affect fertility?
The odds of pregnancy throughout your lifetime
How can I increase my chances of pregnancy?
Anti-Mullerian hormone
What is a normal AMH level?
When should I see my GP?

WHAT AGE DOES FERTILITY DECLINE?

A woman’s peak reproductive years are between the late teens to the mid-twenties [2]. By age 30, fertility starts to decline — very slowly, at first.

From around age 35 and onwards, this drop in fertility becomes more prominent. And by age 45, it becomes unlikely for a woman to fall pregnant, even with fertility treatment [3]. 

Adapted from British Fertility Society [4].

So, why does fertility decline?

Well, women are born with a finite number of eggs — about one to two million — and by puberty, only about 300,000 remain. Of these, 300–400 eggs will be released during a women’s reproductive years.

But there’s more to it than simply a decreasing supply of eggs.

HOW DOES AGE AFFECT FERTILITY?

As women age, the genetic material in their eggs is more likely to accumulate errors or faulty DNA, which reduces the chances of a viable pregnancy. Women in their 20s will have mostly normal eggs, whereas women in their 40s will have mostly abnormal eggs, making it more difficult to conceive.

Changes in egg quality is one of the primary reasons fertility declines with age, but this is not the only reason.

Factors that contribute to age-related declines in fertility: 

• Changes in egg quality - Women in their late 30s or 40s tend to have more eggs with abnormal chromosomes, making it more difficult to conceive and early miscarriage more likely [4].
• Frequency and efficiency of ovulation — Periods become less regular and frequent as a woman approaches menopause. This usually becomes noticeable in the 40s, but it may occur earlier, especially for women who experience early menopause. 
• Medical conditions — Some disorders that affect fertility are more prevalent with increasing age, such as uterine fibroids and endometriosis.
• Increasing risk of pregnancy complications —Pregnancy later in life comes with a higher risk of complications, like pre-eclampsia. For this reason, if you’re 40 or over, you’re more likely to be offered additional monitoring and consultant-led care. 
• Increasing risk of miscarriage — The risk of miscarriage increases with age. For women under 30, about 1 in 10 pregnancies end in miscarriage. This risk doubles for women in their late 30s. For women over 45, more than half of pregnancies end in miscarriage [5]. 

THE ODDS OF PREGNANCY THROUGHOUT YOUR LIFETIME

Falling pregnant comes down to chance and those odds decrease with age. Having said this, all it takes is one healthy egg to fall pregnant.

So, what do those odds look like? Let’s look at the numbers.

Quality of eggs

On average, a 30-year-old woman has about a 20% chance of getting pregnant per cycle. This is a little more likely than rolling a six on a die. But like rolling a six, some will get it first time, for others, it may take a few more rolls. After 12 throws of the die (one year), most women will land a six [6]. 

However, as a woman ages, these odds significantly reduce. As we know, the reason for this decline is mainly due to egg quality. 

So now, in her early 40s, a woman’s chances of becoming pregnant drop to around 1–5% per cycle. This is more like throwing two dice and landing a six on both — it’s less likely, and you’ll probably need a lot more throws. The difficulty is, the number of throws left (number of viable eggs) at this age is limited. 

Number of eggs

The average age of menopause is 51, but women do not remain fertile all the way to menopause. As time passes, women begin to skip ovulation and miss periods. This phase is known as perimenopause. For the sake of this example, let’s assume a woman ovulates and is potentially fertile to the age of 45.

At age 30, she will have about 180 throws of the dice (or 180 cycles) before she exhausts her supply. 

When she’s 40, she has fewer than 60 throws of the dice, but also, her periods are likely sparse and more irregular.

Tying this all together, trying to roll a double six with limited throws is tricky — but not impossible. As with all things based on chance, it just takes one lucky roll. 

Loaded die

For some women, their dice may be weighted unfavourably, meaning they are less likely to become pregnant or cannot become pregnant naturally, regardless of their age. Infertility is quite common — around one in seven couples have difficulty getting pregnant [7]. 

Most people are advised to visit their GP if they’ve not conceived after a year of trying. But you should see your GP sooner if you’re 36 or over, or if you’re aware of any underlying fertility problems [7].

Our fertility hub provides more information on the main causes of infertility. 

HOW CAN I INCREASE MY CHANCES OF PREGNANCY?

Many of the age-related changes that affect fertility are completely natural and inevitable. A lot of it comes down to genetic factors, which can’t be changed. But there are some lifestyle behaviours that you can influence that may increase your chances of pregnancy.

Ways you can increase your chances of becoming pregnant [8]:

• Have regular sexual intercourse, every two to three days ideally
• For some couples, it may be helpful to time sex around the time of ovulation (usually 12 to 16 days before your period starts). However, this can be difficult if a woman’s periods are irregular or cause undue stress, so recent guidance does not specifically recommend this method
• Stop smoking
• Cut down alcohol or stop drinking alcohol altogether
• Strive for a healthy body weight

If you are trying to get pregnant, it’s important to take folic acid every day, eat a healthy diet, and drink no more than two units of alcohol per week. This will help your baby develop healthily. 

Find out more on how you can increase your chances of conception.

Check if you’re ovulating

There are some things that can cause you to temporarily stop ovulating (and therefore unable to conceive), such as extremely heavy exercise [9], high levels of stress, being very underweight, or thyroid disease [10]. These causes are reversible, so once the stressor is removed, ovulation can continue as normal.  

You can check if you’re ovulating normally with an Ovulation Progesterone Blood Test. 

ANTI-MULLERIAN HORMONE 

As the number of eggs decreases with age, so too do levels of Anti-Müllerian hormone (AMH).

AMH is a hormone produced by the follicles in the ovaries. Levels of AMH strongly correlate with the number of eggs you have left in your ovaries, known as ovarian reserve.

What is AMH used for?

AMH is useful to gauge your egg supply but one thing it can’t tell you is the quality of these eggs. As such, AMH is not a reliable indicator of fertility and should be interpreted in the context of age — but it does have its uses.

AMH can:

• Estimate ovarian reserve and tell you whether you might reach menopause early
• Be used as part of egg freezing, as it gives an idea of how many eggs can be obtained in one cycle

Estimating ovarian reserve is particularly relevant for women who are at risk of having fewer eggs, such as having a single ovary, previous ovarian surgery, and previous chemotherapy or radiation.

Knowing whether you might reach menopause early can be useful if you plan on having children in the future. It may mean you bring your family planning forward a few years or consider freezing your eggs.

When it comes to egg freezing (oocyte cryopreservation), AMH is useful in its own right. It gives us an idea of how many eggs can be obtained per cycle. Women who are older have a lower percentage of genetically health eggs, so they must freeze more eggs than younger women for a better success rate.

AMH can’t:

• Accurately predict your chances of conception
• Tell you about egg quality or chromosomal damage to eggs (there is no test for this)
• Tell you how quickly your egg count is decreasing — the test gives only a snapshot in time
• Identify conditions that may affect fertility, such as fibroids or uterine scarring

You can check your AMH levels at home with our Anti-Mullerian Hormone (AMH) Blood Test.

What is a normal AMH level?

AMH levels should be interpreted in context. Having normal AMH levels for your age does not necessarily mean your chances of getting pregnant are very good, as it looks at the overall number rather than quality of eggs, and it does not take other causes of infertility into account.

Occasionally, a condition called polycystic ovarian syndrome (PCOS) can mask a low AMH level, producing a seemingly normal result. 

This graph can help you to understand how levels of AMH change over time.

Adapted from Pearson et al. 2016 [11].

What does a high AMH level mean? 

Having a high AMH level does not necessarily mean you are more fertile. In fact, it could mean the opposite is true. 

Raised AMH levels may be a sign of polycystic ovarian syndrome (PCOS) which can reduce your chances of becoming pregnant. PCOS is usually diagnosed based on symptoms (including irregular periods and infertility), high levels of male hormones, and/or an ultrasound scan. 

If you think you might have PCOS, you should consult your doctor who will advise you on the next steps. Read our PCOS guide for more information.

What does a low AMH level mean? 

Low levels of AMH may point towards a lower ovarian reserve. This does not mean you can’t fall pregnant, but it might mean you’re more likely to reach menopause earlier. If you are keen on having children, you may want to consider bringing your family plan forward. Egg freezing is also another option for women wanting to have children in the future. 

Bear in mind, that a low AMH level may be masked by PCOS. This is one of the limitations of an AMH test. 

How can I increase my AMH levels?

As AMH is just a reflection of the number of eggs in your ovaries, there are no medications or steps you can take to increase this. Your ovarian reserve is fixed from birth. 
However, making healthy lifestyle changes like those listed above may improve your chances of conception. 

When should I see my GP?

Assuming you don’t have any symptoms or risk factors of infertility, you can start trying to get pregnant naturally. If you’re aged 36 or over and want to get pregnant, it’s generally best to see the GP first. They can discuss your overall health with you, your medications, and the risks associated with a later pregnancy.

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References

1. British Fertility Society. n.d. A guide to fertility - Key facts about fertility. [online] Available at: <https://www.britishfertilitysociety.org.uk/fei/#link3> [Accessed 25 January 2022].
2. Hertility Health. 2020. Age and Fertility: Is Your ‘Biological Clock’ Ticking Away?. [online] Available at: <https://hertilityhealth.com/blog/age-and-fertility-is-your-biological-clock-ticking-away/> [Accessed 6 December 2021].
3. ACOG. 2020. Having a Baby After Age 35: How Aging Affects Fertility and Pregnancy. [online] Available at: <https://www.acog.org/womens-health/faqs/having-a-baby-after-age-35-how-aging-affects-fertility-and-pregnancy> [Accessed 25 January 2022].
4. British Fertility Society | BFS. n.d. At what age does fertility begin to decrease? - British Fertility Society | BFS. [online] Available at: <https://www.britishfertilitysociety.org.uk/fei/at-what-age-does-fertility-begin-to-decrease/> [Accessed 25 January 2022].
5. NHS. 2018. Miscarriage - Causes. [online] Available at: <https://www.nhs.uk/conditions/miscarriage/causes/> [Accessed 25 January 2022].
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7. NHS. 2020. Infertility. [online] Available at: <https://www.nhs.uk/conditions/infertility/> [Accessed 25 January 2022].
8. NHS. 2018. How long does it usually take to get pregnant?. [online] Available at: <https://www.nhs.uk/pregnancy/trying-for-a-baby/how-long-it-takes-to-get-pregnant/> [Accessed 25 January 2022].
9. Hakimi O, Cameron LC. Effect of Exercise on Ovulation: A Systematic Review. Sports Med. 2017 Aug;47(8):1555-1567. doi: 10.1007/s40279-016-0669-8. PMID: 28035585.
10. Women and Infants, n.d. Anovulation. [online] Available at: <https://fertility.womenandinfants.org/services/women/anovulation> [Accessed 25 January 2022].
11. Pearson K, Long M, Prasad J, Wu YY, Bonifacio M. 2016. Assessment of the Access AMH assay as an automated, high-performance replacement for the AMH Generation II manual ELISA. Reprod Biol Endocrinol. 14:8. doi: 10.1186/s12958-016-0143-3. PMID: 26879773; PMCID: PMC4754992.