Fetal Growth Week By Week | What Changes Each Week

Fetal Growth Week By Week sounds straightforward until you try to match an app, a scan report, and your own calendar. This article keeps it grounded. You’ll get the weekly milestones people ask about most, plus a clear way to read growth notes without spiraling.

Most clinics count pregnancy by gestational age, starting from the first day of the last menstrual period. Conception often happens about 2 weeks later. That’s why “week 4” can feel early: the body is counting before a test even turns positive.

Fetal growth week by week with trimester landmarks

Weeks help you track details. Trimesters help you see the arc. The pattern is simple: weeks 1–12, then 13–27, then 28–40. Each trimester has its own growth “theme.”

• Weeks 1–8: foundations form fast. The heart, brain, spinal cord, and limb buds start taking shape.
• Weeks 9–20: parts get refined. Fingers, toes, facial features, and organ function become clearer.
• Weeks 21–40: strength and finishing work. Weight gain rises, lungs mature, and the brain builds connections for life after birth.

For a clinician-written overview that tracks these phases, ACOG lays out the big milestones in How Your Fetus Grows During Pregnancy.

Weeks 1–4: Before it feels real

Week 1 is the start of the calendar count, and it lines up with a normal period. Week 2 is the lead-up to ovulation. Around week 3, fertilization may occur, and the earliest cell divisions begin. By week 4, implantation often happens, and pregnancy hormones rise enough for some tests to pick it up.

What matters in this window is organization. Early tissues form layers that later become organs. You won’t feel that work happening. A scan may also show little this early, and that can be normal.

Weeks 5–8: The fastest build

By week 5, early heart activity may be seen on ultrasound, based on dating and scan timing. The neural tube that becomes the brain and spinal cord is forming. Tiny buds show where arms and legs will develop.

Weeks 6–7 bring early facial structure, a stronger heartbeat rhythm, and more defined limb buds. By week 8, many core structures are in place, and the shape looks more recognizably human. From week 9 on, medical naming often shifts to “fetus,” since the basic body plan is set.

Weeks 9–20: Changes you can map to a scan

From week 9 onward, growth is easier to picture. Fingers and toes separate. Eyelids form and stay closed for a stretch. Bones start hardening from cartilage. The placenta also takes a bigger role in hormone production as the first trimester closes.

Movement begins long before most parents feel it. You may hear that motion starts in the first trimester. Feeling it is another story. Many people notice the first flutters later, often around weeks 18–20.

The NICHD pregnancy overview notes that fetal movement is often felt around week 20 and that the mid-pregnancy anatomy ultrasound is commonly done between weeks 18 and 20 (NICHD pregnancy factsheet).

Weeks 9–12: Features sharpen

In weeks 9–10, the head is still large relative to the body. The neck becomes more defined. Tooth buds form under the gums. Hands and feet look less like paddles and more like small limbs.

By weeks 11–12, fingers can bend, nails begin to form, and the abdomen has more space as the body lengthens. If you have a first-trimester scan, this is often the window where dating is confirmed with the most precision.

Weeks 13–16: A steadier stretch

Weeks 13–16 often feel calmer for many people. The skeleton keeps hardening, the face looks more proportionate, and the fetus practices swallowing. Breathing-like motions also begin as the lungs develop, with no air involved yet.

Skin is thin at this point. Fine hair (lanugo) starts to appear later in this stretch. It’s part of how the skin and body surface develop in the fluid-filled space.

Weeks 17–20: The anatomy scan window

By weeks 17–20, many organ systems can be seen in more detail on ultrasound. Measurements of the head, abdomen, and femur help estimate growth. The scan also checks placenta location and amniotic fluid.

Movement becomes more coordinated. Early kicks can feel like taps, bubbles, or a fish turning. They can be subtle at first, then show up in a pattern as weeks pass.

Week-by-week milestones at a glance

This table groups the weeks into practical chunks. Sizes and weights vary by genetics, pregnancy dating, and measurement method. Ultrasound estimates use formulas, so small measurement differences can shift the estimate.

Week range	Common growth notes	Milestones you may hear about
1–4	Implantation begins; early tissues organize	Positive test timing; hormone rise begins
5–6	Early heartbeat activity; limb buds appear	Early ultrasound may show a gestational sac
7–8	Major body plan forms; facial features start	Embryo phase nears its end
9–10	Fingers and toes separate; head still large	Early movements begin, not yet felt
11–12	Nails start; body length increases	Dating scan accuracy is often highest
13–16	Bone hardening continues; features refine	Swallowing practice; lanugo may appear
17–20	Body proportions improve; movement grows	Anatomy ultrasound often done
21–24	Weight gain rises; senses develop	Movement often becomes more consistent
25–28	Brain growth accelerates; lungs keep developing	Many clinics run glucose screening
29–32	More body fat; stronger rolls and stretches	Position may be checked more often
33–36	Rapid weight gain; sleep-wake cycles clearer	Some late-pregnancy tests occur
37–40	Final maturation; growth continues	Term range; monitoring increases

Weeks 21–27: Strength, senses, and steady weight gain

Weeks 21–24 often bring a noticeable jump in movement. The fetus has more muscle tone and more room to push, roll, and stretch. Hearing is developing, and familiar voices may start to register as sound patterns.

Body fat is still building. Skin is less see-through as weeks pass. The lungs keep developing air sacs and surfactant, a substance that helps keep tiny airways open after birth. That work continues well into the third trimester.

By weeks 25–27, the brain is adding folds and making more connections. Eyes can open. Some babies respond to light and sound changes with movement shifts.

When a scan says “small”

Sometimes a report says “small for gestational age” or mentions “fetal growth restriction.” It can land like a punch. It’s also a measurement label. It means the estimated weight is below a percentile cut-off for that week, or the growth curve flattened across visits.

Clinicians often pair that label with extra checks: repeat measurements, amniotic fluid checks, and Doppler studies that review blood flow patterns. The plan depends on the full picture, including placenta function, blood pressure trends, and lab results.

Weeks 28–32: Third trimester growth shifts

Week 28 can change the vibe. Visits may become more frequent. Growth also shifts into “filling out.” Fat and muscle increase, and the brain keeps wiring up networks for breathing rhythm, temperature control, and sleep cycles.

Movement can feel stronger, then a bit different. Space gets tighter, so you may feel fewer sharp kicks and more rolls, stretches, and pressure.

MedlinePlus keeps a straightforward week-by-week outline of fetal development that covers later-week maturation and third-trimester weight gain (MedlinePlus: Fetal development).

Check-ins that track growth and wellbeing

Past the halfway point, tracking often becomes a mix of measurements and pattern-spotting. Here are the check-ins you may see, plus the question each one helps answer.

Timeframe	Common check-in	What it helps answer
Early pregnancy	Dating ultrasound	How far along the pregnancy measures by crown-rump length
Weeks 18–22	Anatomy ultrasound	How structures appear and whether growth tracks expected range
Weeks 24–28	Glucose screening	Whether screening flags follow-up testing for gestational diabetes
After week 24	Fundal height	Whether belly measurement tracks the week-to-week trend
Third trimester	Growth ultrasound in selected cases	Whether estimated weight and fluid level track over time
Late third trimester	Nonstress test or biophysical profile	How heart rate patterns and movement relate to wellbeing
Any time concerns arise	Doppler blood flow study	How blood flow patterns look in main fetal and placental vessels

Weeks 33–40: Filling out, position, and final maturation

In weeks 33–36, weight gain continues and many babies start settling head-down. If the baby stays breech later on, your clinician may talk through options that fit your situation.

In weeks 37–40, the lungs and brain keep fine-tuning, and feeding reflexes like sucking get stronger. Prenatal visits often include more monitoring of blood pressure, swelling, fetal position, and movement patterns.

How to read week-by-week info without stress

Weekly milestones are useful. They can also trip you up when your scan report uses different wording than your app. These ground rules keep things calmer:

• Dating can shift early: An early ultrasound may adjust the due date, which shifts every “week” label that follows.
• Percentiles are a range: A baby can be healthy at many percentiles. A growth curve across visits often tells more than one number.
• Estimates vary by method: Ultrasound uses measurements and formulas, not a scale.
• Ask for the next step: If a note mentions size, fluid, or placenta position, the plan matters more than the label.

A weekly note you can keep

If you like a practical way to track what you notice, keep a short weekly note. It helps you speak clearly at appointments.

• Week number and due date you’re using
• Movement pattern: when it’s most active, what feels normal for you
• New symptom that changes daily routine
• Questions you want answered next visit

If you want a public-health week-by-week hub that links out to each week’s summary, the NHS runs a pregnancy week guide here: NHS week-by-week guide to pregnancy.