Pregnancy Weight Gain: What to Expect & Why It Happens

Pregnancy is a miraculous journey, filled with anticipation and profound changes. One common and often-discussed aspect of this journey is weight gain. While it’s a completely normal physiological response, understanding why your body gains weight during pregnancy can help alleviate concerns and empower you with knowledge. Our expert pharmacist, Zeinab, breaks down the science behind pregnancy weight gain as part of our Women’s Health series, explaining the hormonal shifts and bodily adaptations that support a healthy pregnancy and a growing baby.

What’s Behind Pregnancy Weight Gain?

When the number on the scale starts going up during your pregnancy, it’s a completely normal physiological response. Your body is undergoing incredible transformations to nurture a new life. This weight gain isn’t just about the baby; it reflects a multitude of changes happening within you, including:

• Approximately 1.5 litres of extra blood
• Around 2–3 litres of extra fluid
• The placenta – a completely new organ
• The growing baby
• Uterine growth
• Breast changes in preparation for lactation
• Metabolic adaptations to support the pregnancy

Key Hormones Behind Pregnancy Weight Gain

The science behind pregnancy weight gain is fascinating, largely driven by five important hormones. Each one plays a distinct role in why you gain weight during pregnancy.

hCG: The Pregnancy Alert Hormone

Levels of Human chorionic gonadotropin (hCG) are one of the first hormones to change, alerting your entire body that you’re pregnant. Levels are at their highest at the beginning of pregnancy, peaking at around week eight. This is the primary cause of nausea and morning sickness in pregnancy.

Progesterone: The Sustainer and Fat Reserve Builder

Progesterone levels rise steadily throughout pregnancy. It’s crucial for sustaining the pregnancy and helps relax your muscles, including the uterus, allowing it to expand. It also helps your body build fat reserves — a key contributor to pregnancy weight gain — ensuring you have enough energy for labour and breastfeeding. The muscle relaxation can also contribute to bloating and constipation.

Oestrogen: The Fluid and Blood Flow Regulator

Oestrogen supports increased blood flow and circulation, aids in fluid retention (essential for the baby’s well-being), and plays a key role in developing milk ducts in your breasts. These levels continue to increase throughout your entire pregnancy.

hPL: The Baby’s Energy Supply Hormone

Human placental lactogen (hPL) is produced by the placenta and ensures the baby receives a constant supply of energy from glucose and oxygen. It makes the mother’s body more resistant to insulin, increasing glucose availability for the baby’s growth and development.

Relaxin: The Joint and Muscle Relaxer

As its name suggests, relaxin helps relax joints and muscles throughout your body, preparing it for labour. This is often cited as the reason for “pregnancy butterfingers” – that increased clumsiness from relaxed ligaments!

Blood, Metabolism, and Fluid: Other Causes of Pregnancy Weight Gain

Beyond hormones, several other significant changes contribute to pregnancy weight gain:

• Increased blood volume: During pregnancy your blood volume increases by up to 1.5 litres. This extra blood alone accounts for about 1.5 to 2 kg of weight gain, vital for transporting nutrients and oxygen to your baby.
• Higher metabolic rate: Your body burns 15–20% more energy even at rest, requiring an additional 300–500 calories per day, especially in the third trimester. People sometimes overestimate the additional calories required, and this can lead to weight gain.
• Fluid retention: Your body retains around 6–8 litres of extra water, accounting for 2–3 kg of extra weight. This often leads to swelling in the hands and feet.

Pregnancy Weight Gain by Trimester

First Trimester: The Foundation (Weeks 1–12)

Pregnancy weight gain in the first trimester is minimal. The fertilised egg implants, the placenta begins to form, and the baby’s major organs – lungs, brain, liver, and heart – are rapidly developing. hCG and progesterone levels surge. Despite these dramatic changes, the baby is still tiny (smaller than a lime by week 12), so you don’t need significant extra calories yet. Your body mainly needs rest.

Second Trimester: The Growth Phase (Weeks 13–26)

The baby grows rapidly, the pregnancy becomes more visible, and the placenta is fully grown and producing its own hormones. Blood volume increases significantly. Most women see noticeable pregnancy weight gain during this phase — around 5–7 kg. You may notice a shift in your centre of gravity, which changes the way you walk.

Third Trimester: The Final Stretch (Weeks 27–40)

Your baby grows fastest now, potentially doubling or tripling in size. Your body actively stores energy for labour and breastfeeding. Blood volume and fluid retention continue to increase. Pregnancy weight gain accelerates here, with most women adding 4–6 kg. Fluctuations on the scale are common due to fluid retention.

Managing Pregnancy Weight Gain: Nutrition Tips

For those with a healthy BMI before pregnancy, expect to gain around 10–20 kg throughout pregnancy. If you have a higher BMI, your body already has reserves, so weight gain might be closer to 10 kg. The goal is never to restrict pregnancy weight gain – it’s to ensure healthy development for you and your baby.

The key principles are simple: don’t go hungry, avoid sugar spikes, and keep healthy snacks on hand throughout the day.

First Trimester Tips

• Eat small, frequent meals to manage nausea
• Take your folic acid supplement
• Try ginger tea, toast, and crackers to soothe your stomach

Second Trimester Tips

• Increase iron intake – meat, lentils, and leafy greens. If you need a supplement, ferrous sulfate tablets are available from our pharmacy
• Consume vitamin C alongside iron to aid absorption
• Boost calcium intake with dairy products
• Include oily fish or omega-3 supplements for your baby’s brain development

Third Trimester Tips

• Prioritise fibre and fluids to prevent constipation
• Cooked fruit can be a helpful natural laxative
• Continue with smaller, frequent meals

Important: Stop consuming alcohol and smoking as soon as you know you’re pregnant. There is no safe dose of alcohol during pregnancy.

Remember, regular check-ups with your healthcare provider are essential to monitor your pregnancy weight gain and address any specific concerns. Your body’s response to pregnancy is unique, and personalised advice can make all the difference.

Disclaimer: This blog post is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional for any health concerns or before making any decisions related to your health or treatment.