Cortisol, fertility and pregnancy: what changes and what the evidence shows

Cortisol rises steadily through a healthy pregnancy, to levels that would point to Cushing's syndrome in someone who was not pregnant.¹ That is normal, and it is the main thing to know about cortisol in pregnancy: results have to be read with pregnancy in mind, and blood results are pushed higher still by the high oestrogen of pregnancy.² Hair records the rise as well, which makes it a useful research tool for looking back across a pregnancy.³ On fertility, the picture is more modest than many articles suggest. Severe stress may affect ovulation, but studies that measured cortisol itself have mostly not linked it with how quickly women conceive, and no cortisol test is a fertility test.⁴⁵⁶

Why cortisol rises in pregnancy

Pregnancy changes the whole system that controls cortisol, known as the HPA axis (the hypothalamus, the pituitary gland and the adrenal glands). Both cortisol and ACTH, the pituitary hormone that tells the adrenal glands to make it, rise during pregnancy and can reach the levels seen in Cushing's syndrome.¹ The placenta is thought to be part of the reason: it can make corticotrophin-releasing hormone (CRH) and ACTH of its own, and the mother's pituitary may become less responsive to cortisol's usual brake.¹

The rise builds as pregnancy goes on. In 21 women followed through pregnancy, cortisol in hair and the daily total of cortisol in saliva rose together, and the two measures were correlated throughout.⁷ It is one reason medical sources list pregnancy among the things that change cortisol levels, alongside stress, exercise and some medicines.⁸ For the basics of the hormone itself, see what is cortisol.

Why blood cortisol results mislead in pregnancy

About 90 per cent of the cortisol in blood is bound to a carrier protein called corticosteroid-binding globulin (CBG), and a standard blood test measures bound and free cortisol together. The high oestrogen levels of pregnancy raise CBG, so total blood cortisol overstates how much active, free cortisol there is. Saliva and urine tests measure free cortisol and are not affected by CBG in this way.²

This matters mainly for the rare woman who develops a cortisol disorder in pregnancy. Cushing's syndrome in pregnancy is uncommon, and it can be missed because its signs and test results overlap with those of a normal pregnancy.¹ The Endocrine Society recommends using 24-hour urine cortisol, and not dexamethasone suppression tests, when first checking a pregnant woman for Cushing's syndrome.⁹ That is a decision for the specialist team. A cortisol result in pregnancy only means something when it is read by someone who knows how pregnancy shifts it. For how the urine test works, see the 24-hour urine cortisol test.

What hair shows across a pregnancy

Hair grows about 1 cm a month, so successive 3 cm lengths of hair hold roughly successive three-month periods.¹⁰ That makes it possible to look back across a pregnancy from one sample taken after the birth. In a study of 103 women who had just given birth, the 3 cm of hair nearest the scalp, which grew during roughly the third trimester, held twice as much cortisol as the same segment from 20 control women.³

The same study shows the limits of looking back. Cortisol fell by 30 to 40 per cent from one 3 cm segment to the next as the hair got further from the scalp, and the authors concluded that a hair calendar probably reaches back only about six months.³ So hair shows the late-pregnancy rise clearly, but it cannot reliably compare the first trimester with the last. See segmental hair cortisol analysis for how hair is cut into time periods.

There is a practical point for anyone testing after a birth. The centimetre of hair nearest the scalp reflects hair formed roughly 1.3 to 2.2 months before it was cut, because new hair takes time to emerge and scissors leave a short stub.¹¹ So a sample taken in the first couple of months after giving birth still largely reflects late pregnancy, and a high result then is expected.

Stress in pregnancy and hair cortisol

It is natural to wonder whether worry or low mood in pregnancy shows up in cortisol. For hair, the answer so far is: not much. A meta-analysis of 29 studies found that the overall link between psychological distress in pregnancy or soon after birth and hair cortisol was small and not statistically significant.¹² That fits the wider research, in which hair cortisol does not consistently track how stressed people say they feel.¹³

So a hair test cannot measure how anxious or low you are in pregnancy, and a higher result during pregnancy is expected anyway. If you are struggling, talk to your midwife, health visitor or GP: how you feel matters in its own right, whatever a cortisol test would say. Our guide to cortisol, anxiety and depression covers the wider research.

Can stress or cortisol affect fertility?

Trying to conceive can be stressful in itself, and infertility clearly causes distress. Whether stress causes infertility is much less clear.¹⁴ The NHS says stress can affect your relationship and lower sex drive, and that in severe cases it may also affect ovulation and sperm production.⁴

Studies that measured stress hormones directly point away from cortisol as the main link. In 274 women in the UK who were trying to conceive, those with higher levels of salivary alpha-amylase, an enzyme used as a marker of the body's fast, adrenaline-driven stress response, had a lower chance of conceiving across the fertile days of their first cycle. Salivary cortisol showed no such link.⁵ In a US study that enrolled 501 couples, women in the top third for alpha-amylase had a 29 per cent lower chance of conceiving in each cycle and about twice the risk of infertility (relative risk 2.07), but again salivary cortisol was not linked with how quickly women conceived.⁶ Both studies relied on saliva collected on one or a few mornings, which captures only a moment.

For fertility treatment, a meta-analysis of 14 studies covering 3,583 women found that anxiety or depression before a cycle of assisted reproduction, such as IVF, was not associated with whether it worked. The authors concluded that this should reassure women that distress will not compromise their chance of pregnancy.¹⁵ One small study did look at long-term cortisol. In 88 women about to start IVF at an English clinic, hair cortisol measured beforehand predicted clinical pregnancy, with higher levels going with a lower chance, while repeated saliva measures did not. The authors described this as preliminary evidence.¹⁶

Put together, severe stress can affect fertility, and a 2018 review noted that psychological support during fertility treatment lowers distress and has been associated with higher pregnancy rates.¹⁴ But the evidence does not show that cortisol is how everyday stress affects conception, or that lowering it improves the odds. If you have been trying for a year without success, or sooner if you are 36 or over, the NHS advises seeing a GP.⁴ For the wider picture on stress, see cortisol and stress.

What a cortisol test can and cannot tell you

No cortisol test, whether hair, blood, saliva or urine, is a test of fertility, and nothing in this research is a reason to change your pregnancy care. If you have symptoms that worry you in pregnancy, your midwife or doctor will decide whether any test is needed.

A hair test answers a narrower question: what has my average cortisol been over the last few months? It is the most accurate way to measure that from a single sample, and far more consistent from one test to the next than the alternatives. In one study the same people's hair results correlated at r = 0.84 from month to month, against 0.38 to 0.61 for saliva from week to week.¹⁷ If you are planning a pregnancy after a demanding stretch at work or a difficult year, it can show whether that period has shown up in your cortisol. What it cannot tell you is whether that level affects your fertility, which the evidence does not establish.

During pregnancy and in the first months after the birth, a hair result reflects pregnancy's normal rise, so it cannot be compared with ranges drawn from people who were not pregnant.³ For how to read a result, see hair cortisol results explained, and for tests arranged through the NHS or a doctor, see getting a cortisol test from your doctor.