At 17 weeks your baby is about 13 cm and 150 grams — pear-sized — and has started building a tissue it will depend on the moment it is born: brown fat.
The short answer: brown fat begins developing this week, concentrated around the neck and shoulders, and it is how a newborn generates heat before it can shiver effectively. Sweat glands are forming and the umbilical cord is thickening to carry more nutrients. For you, appetite often surges sharply — a genuine increased need, not a lack of discipline — and heartburn commonly starts as the growing uterus presses on the stomach.
Your baby this week
Weight gain this week is not only about growing longer — brown fat is starting to form, a metabolically active tissue concentrated around the neck, between the shoulder blades and along the spine. Its job after birth is to generate heat directly by burning energy, which matters because newborns cannot shiver effectively to warm themselves the way older children and adults do.
Sweat glands are beginning to develop across the skin, and the umbilical cord is thickening and strengthening — it needs to carry an increasing volume of nutrients, oxygen and waste as the baby grows, and a thicker, stronger cord is more resilient to compression.
Your body this week
Appetite often becomes noticeably stronger around now, sometimes described as ravenous. That is a normal response to genuinely increased demand — the baby is growing quickly, and your own body is still building blood volume and tissue. Aim to meet the hunger with food that builds rather than just spikes blood sugar: protein, fibre, whole grains, alongside whatever you are craving.
Heartburn commonly begins around this week. Two things are working against you at once: the growing uterus is starting to press upward on the stomach, and progesterone relaxes the valve between the stomach and oesophagus that normally keeps acid down. Our heartburn guide covers which antacids are appropriate in pregnancy and the eating changes that help most.
Chloasma — darker patches of pigmentation on the face — may start appearing, driven by the same hormones and made worse by sun exposure. Daily sunscreen limits how pronounced it becomes.
Tests and appointments
Nothing routine falls exactly at 17 weeks, but the quad test window (15–20 weeks) is still open if earlier screening was missed, and fundal height measurement typically continues at each appointment from here.
✅ Worth doing this week
- Eat protein and fibre alongside cravings rather than refined carbohydrate alone
- Avoid lying down for 1–2 hours after eating if heartburn has started
- Apply sunscreen daily to limit chloasma, even on cloudy days
- Do not restrict food to control weight — the second trimester is when the baby needs the most from your diet
- Ask about antacids rather than reaching for whatever is in the cabinet — not all are appropriate in pregnancy
When to call your doctor
Call the same day if: you have bleeding, severe or one-sided abdominal pain, a severe headache with visual disturbance or sudden swelling of the face and hands, fainting, a temperature above 38°C, or severe itching without a rash. Chest pain is different from ordinary heartburn and should always be assessed rather than assumed.
FAQ — 17 weeks pregnant
What is brown fat and why does it matter?
A special type of fat that generates heat by burning energy directly, rather than storing it like ordinary white fat. Newborns cannot shiver effectively, so brown fat — concentrated around the neck, shoulder blades and along the spine — is how they keep warm in the first weeks after birth. It starts developing this week and continues accumulating through the rest of pregnancy.
How big is the baby at 17 weeks?
About 13 cm crown to heel and roughly 150 grams — pear-sized. The umbilical cord is thickening and strengthening as it needs to carry a growing volume of nutrients and oxygen.
Why do I suddenly want to eat everything?
A sharp rise in appetite around 17 weeks is common and reflects genuine increased need — the baby is growing quickly and your own blood volume and tissue are still expanding. It is normal, not a lack of willpower. The practical response is to eat when hungry but favour protein, fibre and whole foods over refined carbohydrates, so the extra calories build rather than just spike blood sugar.
What is chloasma?
Also called melasma or the "mask of pregnancy" — patches of darker pigmentation, usually on the cheeks, forehead, nose or upper lip, caused by pregnancy hormones increasing melanin production. Sun exposure makes it worse, so daily sunscreen genuinely helps limit it. It typically fades within a year after birth, though it does not always disappear completely.
Week 17 at a glance
At 17 weeks your baby is about 13 cm and 150 grams, developing brown fat for warmth after birth, forming sweat glands, and growing a thicker, stronger umbilical cord. For you, appetite often surges — a real increased need rather than a lack of willpower — and heartburn frequently starts as the uterus presses on the stomach and progesterone relaxes its valve. Chloasma, darker facial pigmentation, may also begin, and sunscreen genuinely helps limit it.
Inside Baby Novum: the safe foods guide rates 240 items by trimester, offline, so a sudden 11pm craving can be checked in seconds rather than guessed at, and the weight tracker plots gain against the WHO corridor for your pre-pregnancy BMI.