Guide to pregnancy hormones

Throughout pregnancy, your body becomes flooded with hormones. As pregnancy progresses, you can experience emotional and physical changes, from feeling weepy to experiencing changes to your digestion. 

But what can hormones tell you about how your pregnancy is progressing? If you have just taken our Pregnancy Blood Test or Pregnancy Progress Blood Test, this guide aims to give your results context and further information about each hormone.

1. What happens when an egg is fertilised?
2. What happens to hormones after an egg is fertilised?
3. What can hormones tell me about my pregnancy?
4. Changes during each trimester

What happens to hormone levels when an egg is fertilised? 

After an egg is fertilised, there’s a finely controlled release of hormones that enables the egg to implant and for the pregnancy to progress normally.

Hormones like progesterone, oestrogen, and human chorionic gonadotrophin (hCG) peak and trough throughout pregnancy, which explains symptoms like nausea and breast soreness, but they’re also important to show how a pregnancy is progressing. 

Beta-hCG (β-hCG)

β-hCG is a hormone released by the placenta, and levels rise about six days after fertilisation — this is the hormone that pregnancy tests detect as it is one of the earliest hormones to rise. Levels of hCG typically peak between the eighth and tenth week of gestation and then plateau at a lower level for the remainder of the pregnancy.

hCG supports the corpus luteum to release progesterone towards the beginning of pregnancy as well as other important functions. The corpus luteum is a bundle of cells that forms in the ovaries immediately after an egg is released. It degenerates unless the egg is fertilised.

Progesterone

Progesterone levels steadily increase throughout pregnancy from the first few weeks after fertilisation and peak just before term. Progesterone is produced by the corpus luteum for the first ten weeks, after which the placenta takes over.

Progesterone is responsible for preparing the endometrium (lining of the womb) before the implantation of the fertilised egg. For the remainder of the pregnancy, progesterone plays a role in the development of the foetus, modulation of the maternal immune system, growth of maternal breast tissue, and strengthening of the pelvic floor muscles in preparation for labour. 

Oestrogen 

Oestrogen follows a similar trend to progesterone, rising throughout pregnancy and peaking in the third trimester. It too is produced by the corpus luteum initially. The rapid changes in oestrogen levels are to blame for symptoms like morning sickness.

Oestrogen controls and stimulates the release of other hormones important in pregnancy. It supports the placenta to grow and promotes breast growth and milk duct development. Oestrogen is also ;vital to allow the foetus to develop and mature normally. 

Prolactin 

During pregnancy, prolactin levels increase by up to 20 times. High prolactin levels trigger the body to make milk for breastfeeding. Prolactin levels remain high after labour if the mother continues to breastfeed, otherwise, levels fall quickly. 
 

What can hormones tell me about my pregnancy?

Human chorionic gonadotrophin (hCG)

Levels of hCG typically increase exponentially from day six after fertilisation and peak by the 11th week. Past this point, hCG becomes a less useful way to monitor a pregnancy.

In the first trimester, increasing levels of hCG usually mean that pregnancy is healthy. The upwards trend is more important than the numbers themselves as ranges vary considerably. Typically, levels double roughly every two to three days in early pregnancy.

Low hCG levels may indicate a problem but are not always a sign that something is wrong. 

Causes of hCG levels that are lower than expected: 

• Miscalculated gestational age — This happens when the estimated stage of pregnancy is wrong. Gestational age is based on the last menstrual period, so miscalculations can occur if someone has irregular periods or is not sure when their last period was.
• Blighted ovum — A blighted ovum is a type of very early miscarriage, where the egg implants but stops developing early on and the tiny embryo is reabsorbed. As it occurs within a few weeks, many women aren’t even aware they were pregnant in the first place. It can be reassuring to know that the odds remain very good of having a healthy pregnancy in the future.
• Ectopic pregnancy — In the UK, around one in 90 pregnancies is ectopic. This is where the embryo attaches anywhere outside of the uterus, usually in the fallopian tubes. Unfortunately, an ectopic pregnancy can be dangerous and cannot progress. Levels of hCG tend to be lower with an ectopic pregnancy and rise more slowly than in a normal pregnancy. 
• Miscarriage — A low hCG level may signal that a miscarriage has happened or is about to happen. A miscarriage is the loss of a pregnancy during the first 23 weeks, and they are very common, affecting about one in eight pregnancies. For most women, a miscarriage is a one-off event, and they can go on to have a successful pregnancy in the future.

Causes of hCG levels that are higher than expected: 

• Multiple pregnancy — High hCG levels may be raised due to twins or triplets. A multiple pregnancy is more common after in vitro fertilisation (IVF). 
• Miscalculated gestational age — If the presumed date of the last menstrual period is wrong, the pregnancy could be further along than expected and hCG levels may be unexpectedly raised.
• Molar pregnancy —Molar pregnancies are where the baby and placenta do not develop as they should after fertilisation. Molar pregnancies are very rare and happen by chance. They cannot progress and usually end on their own, with a miscarriage. Others may be detected at a 12-week ultrasound scan. 

Progesterone

Abnormal progesterone levels during pregnancy are uncommon. However, if levels are significantly lower than expected, it may mean the uterus is not able to carry the baby to term. 

Bear in mind that a single blood test in isolation is not always reliable because progesterone levels fluctuate over 24 hours. So, taking the average of several results is a better way to track progesterone levels.

Progesterone levels that are lower than expected may indicate: 

• Ectopic pregnancy — This is if a fertilised egg implants anywhere other than the uterus. It can result in a positive pregnancy test, but the pregnancy will not be able to progress. Progesterone and hCG levels are lower than usual in ectopic pregnancies.
• Miscarriage — Women who have experienced miscarriage tend to have lower levels of progesterone, but it’s not clear which causes which. However, studies have shown that taking progesterone supplements does not affect the outcome in these instances.

Progesterone levels that are higher than expected may indicate:

• Multiple pregnancy — When more than one baby is growing, it can cause hormone levels to increase. A multiple pregnancy usually requires additional monitoring throughout pregnancy as the risk of complications is higher than a single birth. 
• Molar pregnancy — As well as causing raised hCG levels, molar pregnancies can also cause progesterone levels to surge. Finding out about a molar pregnancy can be very difficult. A doctor or midwife can direct you to advice and support that’s available. 

Oestrogen

Though progesterone is considered the dominant hormone during pregnancy, oestrogen is just as vital for a healthy pregnancy. 

Oestrogen plays a role in regulating progesterone release, and a significant drop could signify a miscarriage. However, there is not usually a need to check oestrogen during pregnancy as it may cause unnecessary worry. 
 

Changes during each trimester

First trimester changes (0–12 weeks)

A lot of the symptoms in the first trimester are due to a surge in hCG levels. These include:

• Headaches
• Irritability
• Fatigue
• Insomnia
• Morning sickness

Second trimester changes (13–27 weeks)

In the second trimester, oestrogen and progesterone levels increase and can cause:

• Mood swings
• Nasal congestion and headaches
• Swollen hands and feet
• Hyperpigmentation (darkening) of the nipples and along the midline of the belly
• Breast enlargement 
• Aching back, pelvis, and hips

Third trimester changes (28 weeks and onwards)

Oestrogen and progesterone reach their peak during the third trimester. Symptoms during this period of pregnancy include:

• Indigestion, heartburn, and constipation
• Aches and pains in the back, pelvis, and hips
• Breast growth (the breasts may start to leak colostrum)
• Puffiness
• Varicose veins
   

Summary 

Pregnancy is a unique and individual experience. As such, great care should be taken when interpreting results — something that may appear abnormal may be harmless, and vice versa. Discuss your results with a doctor if you’re concerned. 

There is no right or wrong way to feel about being pregnant, but it’s common to feel vulnerable, anxious, or overwhelmed. If this is the case, speak to your doctor or midwife who can explore these feelings with you and hopefully reassure you. 

If you have lost a pregnancy, The Miscarriage Association is a UK-based charity that offers information and emotional support to help you through a difficult time. 

References

1. NHS. 2021. Doing a pregnancy test. [online] Available at: <https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/> [Accessed 2 December 2021].
2. Cole L. A. 2010. Biological functions of hCG and hCG-related molecules. Reproductive biology and endocrinology : RB&E, 8, 102. https://doi.org/10.1186/1477-7827-8-102
3. Kumar, P., & Magon, N. 2012. Hormones in pregnancy. Nigerian medical journal : journal of the Nigeria Medical Association, 53(4), 179–183. https://doi.org/10.4103/0300-1652.107549