Week 17: Fat Storage Begins

This week, an important process begins — your baby is starting to put on weight! A thin layer of brown fat is forming under the skin, and this fat plays a crucial role in heat regulation after birth. Your baby is now about 13 cm long (crown to rump) and weighs around 140 grams — the size of a pear.

You are well into the second trimester, and many experience this as the most comfortable period of pregnancy. The anatomy scan is approaching, and there is much to look forward to. The anatomy scan is typically offered between weeks 17 and 19 and is a thorough examination of your baby's development.

Calcium needs are especially important now as your baby's skeleton is hardening. According to ACOG and WHO, pregnant people need around 1,000 mg of calcium daily to support your baby's bone development.

Baby's Development

• Brown fat forming: A special fat layer is building up under the skin. Brown fat produces heat and is essential for your baby's ability to regulate body temperature after birth
• Skeleton hardening: The skeleton is gradually transitioning from soft cartilage to harder bone. Calcium is actively being stored in the small bones
• Umbilical cord growing: The umbilical cord is becoming thicker and stronger to transport nutrition and oxygen between you and your baby
• Hearing developing: Your baby is perceiving sounds increasingly well. Your voice and heartbeat are among the first sounds your baby is learning to recognise
• Fingerprints: Your baby's unique fingerprint patterns are almost fully formed
• Meconium: The first bowel movement (meconium) is beginning to accumulate in the intestine
• Movements more coordinated: Your baby can now kick, stretch, and turn with greater precision
• Size: The fetus is about 13 cm long (crown to rump) and weighs around 140 grams

Brown Fat — The Body's Furnace

Brown fat is different from regular white fat. It contains many mitochondria (the cell's power plants) which can convert energy directly into heat without muscle movement. Newborns have proportionally much more brown fat than adults — especially around the shoulders, neck, and back.

It is essential for temperature regulation in the first days after birth, when your baby suddenly needs to maintain its own body temperature instead of being warmed by yours. Brown fat can produce heat up to 300 times more efficiently than ordinary body tissue. Newborns have about 5% of their body weight as brown fat — a vital reserve that gradually decreases during the first year of life.

Your Body

• Balance shifting: Your centre of gravity shifts as your bump grows, and you may feel unsteady
• Stretch marks: Some people get the first stretch marks now, often on the abdomen, hips, or breasts
• Sciatica: The uterus can press on the sciatic nerve, causing pain from the lower back down into the leg
• Increased blood volume: Your blood has already increased by around 40%, and you may notice it as a warm feeling and flushing
• Bump clearly visible: The baby bump is now noticeable to most. The uterus is halfway between the pelvis and the navel
• Increased appetite: Many experience stronger hunger and food cravings during this period
• Sweating more: Increased blood volume and metabolism mean many pregnant people sweat more than usual
• Heartburn may start: The uterus is pressing upward against the stomach, and the hormone progesterone is relaxing the sphincter between the stomach and oesophagus

Sciatic Pain in Pregnancy

The sciatic nerve is the body's longest nerve and runs from the lower back down through the back of the leg. During pregnancy, the growing uterus can put pressure on the nerve, causing pain that radiates from the lower back down through the buttock and the back of the thigh.

Sciatic pain affects around 50–80% of pregnant people at some point during pregnancy. The pain can range from a mild ache to sharp and stabbing. Tips for relief:

• Gentle stretching of the hip muscles
• Warm pack on the lower back
• Swimming relieves the back and provides pain relief
• Avoid sitting or standing in the same position for long
• Use a wedge cushion or pillow under your abdomen when sleeping on your side
• Talk to your midwife about a referral to a physiotherapist

Good to Know

Calcium is especially important now as your baby's skeleton is hardening. According to ACOG and WHO, pregnant people need around 1,000 mg of calcium daily. Good sources include dairy products (milk, yoghurt, cheese), broccoli, kale, and almonds. A glass of milk provides about 300 mg of calcium. Talk to your midwife about supplements if you do not get enough through your diet, especially if you do not eat dairy.

Many experience better energy in the second trimester. It is a great time for light exercise like walks, swimming, or prenatal yoga. Regular activity helps with back pain, sleep problems, and improves mood. According to ACOG, pregnant people should aim for at least 150 minutes of moderate activity per week.

Remember to wear shoes with good support, as your balance is changing. Avoid slippery floors and steep stairs where possible. Flat shoes with good cushioning are a better choice than high heels, especially now that the hormone relaxin is loosening your joints.

If you have not already started Kegel exercises, now is the time. A strong pelvic floor prepares your body for birth, prevents urinary leakage, and helps with recovery afterwards.

The Anatomy Scan (Weeks 17–19)

The anatomy scan is offered between weeks 17 and 19. It is voluntary and is one of the most important prenatal checks:

Preparation

• Drink some water beforehand (not too much — a partly full bladder is ideal)
• Bring your partner — this is a wonderful experience to share
• Write down questions you want to ask
• Bring your maternity notes

What Is Examined?

• Brain and head: Size, symmetry, and development of brain structures
• Heart: The four chambers, valves, and blood flow
• Spine: Complete review of the vertebrae
• Kidneys and bladder: Function and size
• Stomach and bowel: Normal position and function
• Arms and legs: Bone length and joints
• Placenta: Position and function
• Amniotic fluid: Volume assessed

If You Already Have an Appointment

If you already have an anatomy scan appointment this week, an exciting experience awaits. The scan typically takes 20–30 minutes, and you will see your baby in detail. The vast majority (about 97%) receive completely reassuring results.

Sex

Many choose to find out the sex at this scan. Let the sonographer know in advance whether you want to know or want it kept secret. It depends on your baby's position — sometimes your baby is hiding, and you may need to come back.

Calcium and Bone Development

Calcium is one of the most important minerals in pregnancy, and needs are especially high now as your baby's skeleton is hardening.

Recommended Intake

According to ACOG and WHO, pregnant people need around 1,000 mg of calcium daily. If you do not get enough calcium through your diet, your body will take calcium from your own bones to supply your baby. It is therefore important to ensure adequate intake.

Good Calcium Sources

Tips for Better Calcium Absorption

• Vitamin D is needed for the body to absorb calcium. ACOG recommends 600 IU (15 micrograms) of vitamin D daily for pregnant people
• Spread calcium intake throughout the day — the body absorbs calcium best in moderate amounts
• Avoid taking calcium and iron supplements at the same time — they inhibit each other's absorption. Wait at least 2 hours between intakes

Heartburn and Digestion

Many pregnant people experience heartburn for the first time around week 17. It is caused by a combination of hormonal and mechanical factors:

Causes

• The hormone progesterone relaxes the sphincter between the stomach and oesophagus
• The uterus is growing and pressing upward against the stomach
• Digestion is slower in pregnancy

Tips That Help (per NHS and ACOG)

• Eat many small meals instead of a few large ones
• Avoid strongly spiced, fatty, and fried food
• Do not lie down straight after eating — wait at least 2 hours
• Elevate the head of your bed by 10–15 cm
• Avoid carbonated drinks
• Chew food well and eat slowly
• Milk and yoghurt can neutralise stomach acid
• Talk to your midwife about safe antacids for severe symptoms

Constipation

Constipation is also common and is caused by the hormone progesterone slowing the bowel, and by any iron supplements you may be taking. Eat fibre-rich food, drink enough water, be active, and try prunes as a natural remedy. Talk to your midwife about gentle laxatives if needed.

Practical Preparations at Week 17

While your energy is good and your bump is not too large, this is a good time to:

• Look into childbirth preparation classes and book a place (many have waiting lists)
• Start planning the nursery and buy the most important equipment
• Think about your birth preferences
• Talk to your employer about parental leave plans

Exercise and Activity

Regular physical activity is important throughout pregnancy. Remember that after week 16, you should avoid exercising while lying flat on your back:

• Walking: 30 minutes daily at moderate pace
• Swimming: Excellent for back and joints — the water supports your weight and relieves your body
• Prenatal yoga: Strengthens and stretches the body, provides mental calm and practises breathing exercises for birth
• Kegel exercises: 3 sets of 10 repetitions daily. Start with short holds and build up gradually
• Adapted strength training: Strength training tailored for pregnancy with focus on back, legs, and pelvic floor

ACOG recommends at least 150 minutes of moderate activity per week. Listen to your body and reduce intensity if necessary. You should be able to hold a conversation while exercising.

Omega-3 and Brain Development

Omega-3 fatty acids, especially DHA (docosahexaenoic acid), are crucial for your baby's brain development. At week 17, the brain is in a phase of rapid growth, and DHA needs are high.

Recommendations

According to ACOG and WHO, pregnant people should eat oily fish 2–3 times per week, where at least one portion is oily fish like salmon, mackerel, sardines, or trout. Fish is the best source of long-chain omega-3 fatty acids.

Good DHA Sources

• Salmon: One of the best sources of DHA. Both fresh and smoked salmon are safe
• Mackerel: Rich in omega-3 (avoid king mackerel due to high mercury content)
• Sardines: Good source of DHA
• Trout: Mild flavour and rich in DHA
• Algae-based omega-3: Good plant-based alternative for those who do not eat fish

Fish to Limit or Avoid During Pregnancy

• Shark, swordfish, king mackerel, tilefish: High mercury content
• Raw fish (sushi): Risk of parasites and bacteria
• High-mercury canned tuna: Best to limit

Mental Health and Preparation

At week 17, many begin to think more concretely about birth and parenthood. It is natural and positive to prepare mentally:

Tips for Mental Preparation

• Talk to your partner about expectations for the birth and postnatal period
• Look into childbirth preparation classes — many fill up quickly, so book early
• Start thinking about your birth preferences
• Read about birth and the postnatal period to feel more confident
• Keep track of important dates with our due date calculator

Birth anxiety can affect up to 10% of pregnant people, according to the NHS and ACOG. If you experience strong anxiety or worry related to birth, talk to your midwife — there is good help available, including dedicated classes for those with birth anxiety.

For Partners

• Offer gentle massages of the lower back and shoulders, especially for sciatic pain
• Come to the anatomy scan — it is a fantastic experience to see your baby together
• Help make the home safe — tidy away things that could be tripped over, as balance changes
• Start discussing practical preparations: nursery, equipment, parental leave division
• Cook meals together with a focus on calcium-rich ingredients

Tips for Week 17

1. Sleep on your side: Continue getting used to side-lying with a pillow between your knees. A good pregnancy pillow can make a big difference
2. Stretch daily: Simple stretching exercises for back and hips prevent sciatic pain
3. Moisturise your bump: Use moisturiser or oil on your abdomen and hips to keep skin soft
4. Eat calcium-rich foods: Choose foods that strengthen your baby's bone development. Combine with vitamin D for better absorption
5. Prepare questions for the scan: Write down what you want to know
6. Document your bump: Take weekly photos
7. Think about your birth preferences: It is never too early to note down your wishes for the birth
8. Follow the pregnancy timeline: Keep up to date with what is happening week by week

When to Contact Your Doctor

Contact your doctor or midwife if you experience:

• Sudden swelling in face, hands, or feet
• Severe, persistent pain in the abdomen or back
• Vaginal bleeding
• Fever above 38°C (100.4°F)
• Dizziness that does not resolve with rest
• Sciatic pain so severe you cannot walk or sleep
• Watery discharge that may indicate amniotic fluid leaking

Sleep and Rest at Week 17

Many pregnant people experience sleep problems during this period. Here are tips for better sleep:

Common Sleep Problems

• Frequent night-time bathroom trips
• Discomfort finding a comfortable sleep position
• Leg cramps and restless legs
• Heartburn that worsens when lying down
• Vivid dreams and nightmares (common in pregnancy)

Tips for Better Sleep

• Sleep on your side (preferably left) with a pillow between your knees
• Use a pregnancy pillow for extra support
• Avoid caffeine after lunch
• Limit fluid intake in the evening (but drink enough during the day)
• Establish a relaxing evening routine
• Keep the bedroom cool and dark
• Gently stretch calf muscles before bed
• Wait at least 2 hours between your last meal and bedtime

When Sleep Problems Should Be Investigated

Talk to your midwife or doctor if you experience:

• Heavy snoring that has started during pregnancy (may indicate sleep apnoea)
• Persistent insomnia affecting daily function
• Severe leg cramps waking you several times a night

Did You Know?

• Brown fat is special: Brown fat contains many mitochondria and can produce heat directly — up to 300 times more efficiently than ordinary body tissue. Newborns have proportionally more brown fat than adults, and it is essential for maintaining a stable body temperature in the first days after birth
• The umbilical cord is an engineering marvel: The umbilical cord contains two arteries and one vein, surrounded by a gel-like substance called Wharton's jelly. This jelly protects the blood vessels from being compressed, even when your baby twists and turns. At full term, the umbilical cord is about 50–60 cm long
• Your baby hears your heart best: Of all the sounds your baby perceives, your heartbeat is the clearest and most constant. That is why many newborns calm down when held close to your chest — the sound is safe and familiar
• Meconium is dark and sticky: Your baby's first bowel movement, meconium, is starting to accumulate in the intestine now. It consists of amniotic fluid, mucus, and skin cells — and is completely odourless because the intestine is still sterile

Frequently Asked Questions About Week 17

What can I do about sciatic pain?

Sciatic pain is caused by the uterus putting pressure on the sciatic nerve. Gentle hip stretching, warm packs on the painful area, and light activities like swimming can relieve it. Avoid standing or sitting in the same position for long. Talk to your midwife or a physiotherapist if pain is severe or persistent.

Is it normal to feel unsteady?

Yes, it is completely normal. Your centre of gravity shifts as your bump grows, and the hormone relaxin makes joints looser and more mobile. Wear shoes with good support, hold the handrail on stairs, and be extra careful on slippery surfaces. Balance improves again after birth when hormone levels normalise.

How much calcium do I need?

According to ACOG and WHO, pregnant people need around 1,000 mg of calcium daily. Good sources are milk, yoghurt, cheese, broccoli, kale, and almonds. A glass of milk provides about 240 mg of calcium. If you do not eat dairy, talk to your midwife about supplements to ensure your baby gets enough calcium for bone development. Remember that vitamin D is needed for calcium absorption.

Can I take a warm bath?

You should avoid very hot water, as it can raise your body temperature too much. Keep the temperature below 38°C and limit the bath to 10–15 minutes. A lukewarm bath is safe and can actually help with back pain and muscle tension. Hot tubs and very hot baths should be avoided in pregnancy, according to ACOG and the NHS.

What if the anatomy scan shows something abnormal?

The vast majority (about 97%) receive completely reassuring results at the anatomy scan. If something is unclear, the sonographer will either ask you to come back for another scan (your baby may have been lying in an awkward position) or refer you to a specialist for a more thorough assessment. Try not to worry in advance — most unclear findings turn out to be completely normal on re-examination.

Should I buy a pregnancy pillow?

A pregnancy pillow can make a big difference to sleep quality from around weeks 16–20 onwards. It supports the abdomen and back, and keeps the body in a side-lying position. A long, C-shaped pillow is most popular. You can also use regular pillows between your knees and under your abdomen as a cheaper alternative. Many find that a good sleep pillow significantly reduces back and hip pain at night.

What is Wharton's jelly in the umbilical cord?

Wharton's jelly is a gel-like substance that surrounds the blood vessels in the umbilical cord. It acts as a natural protection against the blood vessels (two arteries and one vein) being compressed or bent when your baby moves. The jelly makes the umbilical cord elastic and flexible, so blood flow to your baby is maintained regardless of position. At full term, the umbilical cord is about 50–60 cm long and 1–2 cm in diameter — an impressive lifeline between mother and baby.

How much weight should I be gaining per week?

According to ACOG, normal weight gain in the second trimester is about 0.4–0.5 kg per week. Total weight gain depends on your pre-pregnancy BMI. Remember that weight gain is not just the baby — it also includes the placenta, amniotic fluid, increased blood volume, larger breasts, and uterus. Do not weigh yourself daily — weekly is enough. Focus on eating nutritiously and staying active. Weight is also checked at prenatal appointments.

Can I fly at week 17?

Yes, the second trimester (weeks 13–27) is generally considered the safest period for air travel. Most airlines allow pregnant passengers to fly until week 36 (week 32 for twins). Drink plenty of water during the flight, move regularly, and wear compression stockings to prevent blood clots. Talk to your midwife first if you are planning a long trip, and remember to bring your maternity notes.

What if I have an anterior placenta?

An anterior placenta means the placenta has attached to the front wall of the uterus, between your baby and your abdomen. It is a completely normal variation that does not affect your baby's development or health. The only practical consequence is that you may feel fetal movements later than others, because the placenta cushions the kicks. Most people with an anterior placenta feel clear kicks from weeks 22–24. Placental position is reported at the anatomy scan.

Is it safe to paint the nursery while pregnant?

Generally, it is safe to paint in a well-ventilated room with water-based paint. Avoid oil-based paints, varnishes, and solvent-based products. According to ACOG and the NHS, pregnant people should avoid inhaling strong chemical fumes. Let the room air well for several days after painting. If possible, let your partner or others do the painting. Consider using low-VOC paint, which has lower levels of volatile organic compounds.

Follow your whole pregnancy week by week with the pregnancy timeline, and use our due date calculator to keep track of important dates.

Next week: Your baby yawns, hiccups, and stretches — and you may feel the first kicks!