The Effect of Pregnancy on Biomarkers

Introduction – why track biomarkers during pregnancy?

Tamar has generously agreed to share the results of her regular blood tests as well as the wearable data from her Garmin watch, which tracks metrics like heart rate, hours of sleep, and activity levels.

While typical antenatal checks are centred around screening for infectious diseases and blood disorders, Tamar chose to go a level deeper with her testing. Her motivation was mainly to see how markers she was already tracking regularly, such as heart rate and cholesterol, would change throughout pregnancy. Additionally, she wanted to track markers that are linked to foetal development, such as folate, iron and omegas, to ensure she was getting enough of these important nutrients in her diet.

Let’s dive into the data, starting with the most visible change that takes place during pregnancy.

Body weight – how it changes before and after birth

During pregnancy, average weight gain is said to be between 10 and 12.5kg, though this varies by individual [1]. Tamar gained slightly more than the expected 1-2kg in the first trimester, followed by 0.6kg per week in the second and 0.35kg per week in the third. Studies have shown that typical weight gain during pregnancy ranges from 0.1 to 0.9kg per week [2].

Immediate postpartum weight loss was around 5-6kg, which includes the baby, placenta and amniotic fluid. Beyond that, around half of the weight loss occurs in the first six weeks. It's important to avoid heavy exercise or crash dieting during this phase, as the body requires time to recover.

For those who are breastfeeding, the increased energy demands (approximately 500 kcal/day) can support a gradual and natural postpartum weight loss. However, each person's experience is unique, and safe weight management should be guided by overall health and individual circumstances.

In Tamar’s own words: “I had a fairly easy pregnancy and am grateful for that — my biggest struggle was first trimester nausea. I found I had a lot of cravings for Mexican food and cheese sandwiches, which seemed to help relieve this. My weight very quickly shot beyond "recommended", which comes with a graph on Garmin comparing your weight gain to guidelines. I really didn’t want my weight to be in focus, so I tried not to worry and just tried to keep up the nutritious input as well as cravings. In hindsight, I feel it was what my body needed to do — it felt "natural" and not particularly in my control.”

Heart rate – the cardiovascular shift in pregnancy

Resting heart rate normally increases during pregnancy as the body works to pump blood to the organs and placenta, with an average increase of 10% [3]. This was the case here, swiftly rising from a baseline of 55 bpm to 60 bpm, before coming down in the weeks leading up to delivery. The decline in resting heart rate after week 34 is also a common occurrence, often attributed to reduced external stressors, changes in the autonomic nervous system and the cardiovascular system adapting to the higher demands [4, 5]. 

Tamar’s resting heart rate has remained higher than her pre-pregnancy level of 55 bpm, potentially due to reductions in sleep or loss of fitness during pregnancy, and so it may take longer for her to return to her baseline level.

Activity – staying active as the body adapts

As an avid runner, Tamar was out on the roads several hours per week during the early months of her pregnancy, with lower impact activities like walking and swimming becoming more prevalent as things progressed. The Garmin watch data for the third trimester shows the equivalent of 45 minutes per day of walking, dropping to 15 minutes per day from the second month postpartum.

The NHS advice is to continue your normal physical activity during pregnancy for as long as you feel comfortable [6]. Switching to simpler activities like walking allows you to still remain active while reducing the overall strain on the body.

From Tamar: “I started tracking walks because it helped me feel intentional about walking as exercise. I also did a fair amount of strength training that I didn't always track. This was my favourite thing to do — I valued feeling strong! I had the support of a pelvic physio and advice from doctors around continuing to run during pregnancy, and returning to running after, which has been a journey — I've had to go slow. I followed the guidance to wait three months, and have been lucky enough to work with a PT specialising in postpartum functional fitness. I also had childcare, which has supported me to take the time to move!”

Cholesterol – natural rises and what to watch

Cholesterol is a crucial building block for cell membranes and is essential for the rapid growth and development of the foetus. As such, there is a natural rise during pregnancy to support foetal growth. While it is known that cholesterol rises in pregnancy, it is not routinely measured or treated. There are also no established reference ranges for pregnant women, which makes it difficult to diagnose someone as having “high” cholesterol during pregnancy [7].

Tamar saw a 50% increase in total cholesterol (3.7 to 5.6 mmol/L), a doubling of LDL cholesterol (1.4 to 2.8 mmol/L) and a 70% increase in Non-HDL Cholesterol (1.8 to 3.1 mmol/L). Triglycerides also doubled during pregnancy (0.7 to 1.4 mmol/L) but returned to normal very soon after birth. Cholesterol levels can stay elevated for around a month after childbirth, which can be seen in several of her markers tested two weeks postpartum.

While the increases in cholesterol levels align with the research, they can still be a surprise if they’re not something you were expecting to see. Although a moderate rise is expected, very elevated cholesterol levels in the first trimester are associated with adverse outcomes, such as preeclampsia, hypertension and metabolic syndrome [8].

From Tamar:  “I remember reading that cholesterol typically increases during pregnancy but can drop back really quickly after birth, so I tried to do my test as soon as possible to see that effect. It’s a shame we know so little about health in pregnancy and postpartum; it’s really hard to find much beyond very high-level guidance. I'd love to see more research and clinical guidance that's actionable for pregnant people, it’s very hard to know what’s best for your health during pregnancy. A lot of the focus is on the baby, which is obviously great, but I wanted to care for us both!”

Iron – understanding changing needs during pregnancy

Pregnancy significantly increases the body's iron requirements due to the need for more red blood cells to carry oxygen to the growing foetus and the 30-50% higher blood volume of the pregnant woman. 

Iron status is something routinely monitored at around weeks 12 and 28 during pregnancy. The NHS often uses haemoglobin to detect the presence of anaemia, while ferritin is a different biomarker that helps doctors determine whether iron deficiency may be an underlying cause.

Ferritin is often a more sensitive early indicator of deficiency, especially in the first and second trimester, and is a measure of the amount of iron stored in the body. Haemoglobin reflects current oxygen-carrying capacity, but is less sensitive to early iron deficiency.

Tamar’s Ferritin levels moved towards the low end of normal (30 to 200 µg/L) as her pregnancy progressed, and was something she already tracked regularly, given the absence of red meat in her diet.

From Tamar: “During pregnancy I supplemented iron because my haemoglobin was borderline throughout — and I really tried to get more in my pescatarian diet. I was a bit on and off with supplements due to side effects and varying advice from healthcare professionals. I’d really recommend regular tracking here as iron status is so important for how you feel, and if you lose a fair amount of blood during the birth like I did that adds an extra strain that you can feel when recovering”

B vitamins – folate and B12 in pregnancy

B vitamins play a vital role in a healthy pregnancy and are essential for DNA synthesis, cell division and development of the nervous system. The body’s requirements increase to meet the growing demands of pregnancy, and monitoring these levels can reduce the risk of conditions such as neural tube defects.

Women who are pregnant or trying to conceive are recommended to consume a daily folic acid supplement of 400 micrograms up to week 12 of pregnancy [9]. It is important to consult a healthcare professional, as some women may need to take a higher dose of folic acid supplements — for example if you take anti-epileptic medication, have conditions such as diabetes or sickle cell disease, or are a woman with a BMI over 30. The normal range for folate (vitamin B9) is 10 to 30 nmol/L, with optimal up to 45 nmol/L. For active B12, the normal range is between 70 and 150 pmol/L.

Inflammation – what CRP tells us in pregnancy

One of the most common markers of inflammation is C-Reactive Protein (CRP). While a mild increase in inflammation is expected as part of normal immune function during pregnancy, very elevated CRP levels have been associated with conditions such as preeclampsia, preterm birth, and intrauterine growth restriction [10]. It has been established that pregnant women exhibit higher levels of CRP than non-pregnant women, with a median range of 7 to 9 mg/L [11]. 

Tamar started from a very low baseline of 0.25 mg/L, and her levels of inflammation only increased to a peak of 3.07 mg/L in week 16, likely due to her healthy lifestyle prior to her pregnancy.

Regular physical activity and a diet rich in anti-inflammatory foods, such as fruits, vegetables, and omega-3 fats, are known to help keep CRP levels low. Tamar’s relatively modest increase suggests that these habits may have helped buffer some of the typical inflammatory changes seen in pregnancy.

Sleep – tracking rest through pregnancy and beyond

We’ll finish with a measurement that all new parents can relate to — sleep!

Tamar had enviable amounts of sleep during the pregnancy itself, averaging 8 hours of sleep per night in both the second and third trimesters. Then everything changed. The first six weeks after the new arrival saw her average drop to only 5 hours and 25 minutes per night. This was followed by a step change in week 7 which saw her immediately gain an extra hour per night. 

Tamar: “When my daughter was around 7 weeks old, my partner started doing a bottle a day so I could get a reliable block of sleep in, though I still had to wake for expressing milk as she was exclusively on breast milk. We also mixed up who was staying up with her, because she has been very, very tricky to get into a cot. Initially we were doing shifts with her sleeping on us, and we switched how we did those shifts so I got an early block of sleep each evening. This meant I got more deep sleep, according to Garmin!”

Summary – key takeaways from tracking pregnancy

The data that Tamar has shared hopefully gives you a deeper understanding of the changes that take place during and after pregnancy. While every pregnancy is different, tracking these kinds of markers can help us understand our own bodies better and make more informed decisions. 

For anyone currently pregnant or trying to conceive, there are a few considerations that this data provides:

• Establish your health baseline before pregnancy: Tracking pre-pregnancy biomarkers like cholesterol, iron, and resting heart rate gives valuable context for changes during and after pregnancy
• Start folic acid early: In line with NHS recommendations, supplement folic acid while trying to conceive through to the end of week 12
• Proactively monitor iron: Get a ferritin test if you're feeling unusually tired - even if your haemoglobin looks fine. Track iron status regularly, especially if you don’t eat iron-rich foods like red meat
• Track key markers for foetal development: Whether through the NHS or private blood testing, ensure you are aware of any key deficiencies including iron, folate and B12
• Focus on your recovery trajectory - not an immediate return to baseline: Postpartum biomarkers don’t reset overnight. Tamar’s resting heart rate, cholesterol, and activity levels recovered gradually - not immediately. Try not to stress about bouncing back too quickly - track trends over weeks and months, not days.
  

We hope this article has provided insights into a very common but understudied area of the human experience, and helped uncover some of the lesser-known changes that happen to the body during pregnancy. It is important to consult a healthcare professional if you develop any unexpected symptoms during pregnancy as further testing may be required to identify specific pregnancy-related conditions.