Pregnancy Week by Week: What's Happening to Your Body and Baby

So you just found out you're pregnant. Or maybe you've known for a few weeks and you're deep into late-night Googling, trying to figure out what's actually going on in there. Either way, you're in the right place.

Pregnancy runs about 40 weeks — counted from the first day of your last period, not from conception. That catches a lot of people off guard. Your baby was actually conceived around week 3 of the "pregnancy timeline," give or take. Weird system, but it's what doctors use worldwide.

Here's what's happening inside your body — and to your baby — each step of the way. No filler, just the stuff that actually matters.

If you already know your last period date, plug it into our Pregnancy Due Date Calculator to get your estimated delivery date before reading on.

First Trimester (Weeks 1–12): When Everything Starts

Weeks 1–4: Conception and Implantation

Your Baby

Tiny doesn't begin to describe it. We're talking poppy-seed size. After ovulation (roughly week 2), the fertilised egg travels down the fallopian tube and reaches the uterus. Somewhere between 6 and 12 days after ovulation, it burrows into the uterine lining. That's implantation — and that's when your body starts producing hCG (human chorionic gonadotropin), the hormone that keeps everything going.

Your Body

Honestly? You probably won't feel much yet. Some women get a day or two of light cramping or faint spotting when the embryo implants. Most notice nothing. A home test can usually pick up hCG by the end of week 4.

Weeks 5–8: The Heartbeat Weeks

Your Baby (Sesame Seed → Raspberry)

This is when things get real. By week 6, your baby's heart starts beating — around 100–160 beats per minute, way faster than yours. The neural tube closes and becomes the brain and spinal cord. Arm and leg buds appear by week 7. By week 8, fingers and toes are forming (still webbed, but they're there), and almost every major organ system has started developing. At this point, the embryo is roughly the size of a raspberry — about 0.6 inches.

Your Body

And here comes the fun part. Nausea. Fatigue so heavy you could fall asleep standing up. Breast soreness that makes your usual bra feel like sandpaper. Progesterone is surging, and your body is adjusting to a massive hormonal shift. Some women also get weirdly sensitive to smells — coffee, cooking oil, perfume, all of it.

Weeks 9–12: Looking Like a Baby

Your Baby (Grape → Lime)

By week 11, the embryo is officially called a fetus, and external genitalia start developing (though you won't know the sex yet). By week 12, your baby has fingernails, eyelids, and a tiny nose. About 2.5 inches long. If you get an ultrasound around now, you might see them yawning or stretching. Wild.

Your Body

The nausea usually starts easing up around weeks 11–12. You might notice a small bump forming — or not. Every body carries differently. The risk of miscarriage drops significantly after week 12, which is why many parents choose this point to share the news.

Schedule your first-trimester blood work and ultrasound if you haven't already. Keep up with those prenatal vitamins — iron, calcium, and folic acid are non-negotiable right now.

First Trimester Checklist

• Start taking prenatal vitamins daily
• Schedule your first prenatal appointment
• Cut alcohol, smoking, and limit caffeine to under 200mg/day
• Begin tracking symptoms and questions for your doctor

Second Trimester (Weeks 13–26): The One You'll Actually Enjoy

Weeks 13–16: Energy Comes Back

Your Baby (Peapod → Avocado)

Growing fast — about 3–4 inches by week 16. The skeleton starts hardening. Eyelashes, eyebrows, and fine hair are appearing. If this isn't your first pregnancy, you might feel a light flutter of movement around week 16. First-time parents usually feel it a bit later.

Your Body

This is the "golden trimester" and it's earned its name. Nausea fades. Energy returns. Your appetite picks up. You'll likely start showing — a visible bump as the uterus rises above the pelvis. Skin changes are common: a dark line (linea nigra) down your belly, darker areolas, maybe some spots on your face (melasma).

Providers typically recommend about 300 extra calories per day now. That's roughly: a banana, a handful of almonds, and a yoghurt. Not "eating for two" the way your aunt thinks. Track whether your weight gain is on target with our Pregnancy Weight Gain Calculator.

Weeks 17–20: Kicks and the Big Scan

Your Baby (Turnip → Banana)

Your baby can hear sounds now. Talking, music, your heartbeat — they're taking it all in. The mid-pregnancy anatomy ultrasound happens between 18–22 weeks. This scan checks every organ system and usually reveals the sex if you want to know.

Your Body

Quickening — those first unmistakable kicks and rolls — usually hits between 18 and 20 weeks. Your uterus is growing fast. Round ligament pain (a sharp pulling ache on one side of your lower belly) is common and completely normal, though it can catch you off guard. You might get occasional nosebleeds or notice your gums bleed when brushing — blood volume has increased by nearly 50% at this point.

Wondering about your baby's sex? Have a bit of fun with our Chinese Gender Predictor while you wait for the official scan.

Weeks 21–26: The Bump Gets Serious

Your Baby (Carrot → Cauliflower)

Your baby goes from about 10 inches to 14 inches and nearly triples in weight during this stretch. Body fat starts building up, smoothing out wrinkly skin. The amniotic fluid your baby floats in is now being swallowed and recycled — it's actually how the digestive and urinary systems practise working before birth. By week 24, the baby reaches viability — meaning with intensive care, survival outside the womb becomes possible.

Your Body

Daily kicks, rolls, and hiccups become the new normal. Swelling in your ankles and fingers picks up. Stretch marks might appear on your belly or breasts. Heartburn and constipation are likely sticking around thanks to progesterone slowing your digestion.

A glucose tolerance test between weeks 24–28 screens for gestational diabetes. Your doctor will also check iron levels. Start pelvic floor exercises (Kegels) now. Seriously. Your future self will thank you during delivery and recovery.

Second Trimester Checklist

• Complete the anatomy scan (18–22 weeks)
• Take the glucose tolerance test (24–28 weeks)
• Start Kegel exercises daily
• Begin researching pediatricians
• Look into childbirth education classes

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

Weeks 27–30: Brain and Lungs in Overdrive

Your Baby (Cauliflower → Cabbage)

The brain is growing rapidly — forming billions of neurons. Lungs are maturing but aren't quite ready yet. Fat layers build under the skin to regulate body temperature after birth. Your baby responds to familiar voices now. The placenta is working at full capacity, delivering oxygen and nutrients through the umbilical cord while filtering waste back the other way. If born at this stage, survival chances are good with neonatal care, though a few more weeks makes a big difference.

Your Body

Backaches and hip pain become constant companions as your centre of gravity shifts. Sleeping gets harder — side-lying with a pillow between your knees is usually the most comfortable position. Prenatal visits increase to every 2–3 weeks. You'll probably get a Tdap vaccine (whooping cough) around week 28 and a flu shot if it's the season. Both protect your baby after birth.

Weeks 31–36: Nesting Mode On

Your Baby (Coconut → Romaine Lettuce)

Getting big and running out of room. By week 36, most babies settle head-down in the pelvis ("lightening"). The lungs produce surfactant — a substance they need to breathe air after birth. Amniotic fluid levels peak around week 34 and then gradually decrease as the baby takes up more space.

Your Body

Breathlessness. Heartburn. Bathroom trips every hour. Your uterus is pressing against your lungs, stomach, and bladder — all at once. Braxton Hicks contractions (practice contractions) become more noticeable — they're irregular and don't get stronger. If they do get regular and intense, call your doctor. Your cervix starts softening and thinning in preparation for birth.

The nesting instinct is real. You might wake up at midnight with an overwhelming urge to organise the baby's wardrobe. Go with it. Now's the time to finalise your birth plan and pack your hospital bag. Already thinking about names? Browse our Baby Name Finder — you can search by origin, meaning, or even numerology.

Weeks 37–40: Full Term

Your Baby (Swiss Chard → Small Pumpkin)

Fully cooked. Organs are mature. Your baby's gaining about half a pound per week, practising breathing movements, and waiting for the big day. The placenta begins to age and will be delivered shortly after your baby — its job done.

Your Body

Pressure. So much pressure. The baby's head is sitting right on your cervix and bladder. You might notice colostrum (early breast milk) leaking. Your provider will check the baby's position and likely schedule weekly visits.

Watch for these signs of labour: regular contractions about 5 minutes apart, the "bloody show" (pink-tinged mucus), or your water breaking. If any of these happen — it's go time.

Third Trimester Checklist

• Get the Tdap vaccine (around week 28)
• Pack your hospital bag by week 36
• Finalise your birth plan and choose a pediatrician
• Learn the signs of labour vs Braxton Hicks
• Install the car seat — hospitals check before you leave

Baby Size Chart: Week by Week

Want to track your baby's predicted growth? Our Child Height Predictor gives you an estimate of how tall your child might be, and our Baby Blood Type Predictor can help you understand what blood type your little one might inherit.

Sources

• Mayo Clinic — Fetal Development: Week by Week
• ACOG — How Your Fetus Grows During Pregnancy
• NHS — Your Pregnancy Week by Week
• CDC — Folic Acid
• Johns Hopkins — Stages of Fetal Development
• Cleveland Clinic — Braxton Hicks Contractions

This article is for informational purposes and does not replace medical advice. Talk to your healthcare provider about any specific concerns.