Pregnancy Week by Week: Symptoms, Baby Size & Body Changes

Forty weeks. That number can feel both impossibly far away and like it’s racing toward you, whether you’re newly pregnant, planning ahead, or just curious about what each stage brings. This guide breaks down fetal development, your body’s changes, and the NHS-recommended care schedule so you know what to expect — and when.

Stages of pregnancy: 3 trimesters, about 40 weeks ·
Average duration: 280 days from last menstrual period ·
Common first sign: Missed period ·
Major organ development: Weeks 3–8 (embryonic period) ·
Fetal movement felt: Typically weeks 18–22 ·
Full term defined: 39 weeks and later

Quick snapshot

1Confirmed facts

Pregnancy lasts about 40 weeks from LMP (NHS UK (national health service))
hCG rises after implantation (March of Dimes (maternal health organization))
Fetal movement felt between 18–22 weeks (Pregnancy Birth & Baby (Australian Government))

2What’s unclear

Exact due date cannot be guaranteed; only 4% of births happen on the due date (NHS UK (pregnancy calculator))
Severity of morning sickness varies widely by individual (Pregnancy Birth & Baby (Australian Government))
Baby’s gender determination from scan is not 100% accurate (March of Dimes (maternal health organization))

3Timeline signal

Week 6: Heartbeat detected (~105 bpm) (March of Dimes (maternal health organization))
Week 12: Nuchal translucency scan; baby size of a plum (Pregnancy Birth & Baby (Australian Government))
Week 20: Anomaly scan; often learn baby’s sex (NHS UK (national health service))

4What’s next

Book first prenatal appointment by week 8 (NHS UK (national health service))
Schedule dating scan at 8–14 weeks (NHS UK (national health service))
Prepare third-trimester growth scans if needed (March of Dimes (maternal health organization))

Six key benchmarks define the arc of a typical pregnancy, from the first missed period to the moment of birth.

Measurement	Value
Total pregnancy duration	40 weeks (average)
First detectable heartbeat	Around week 6 (vaginal ultrasound)
Fetal movement felt	Weeks 18–22
Baby’s body length at birth	~20 inches
Baby’s average birth weight	7.5 pounds
Start of labor signs	Contractions, water breaking, bloody show

The numbers lay out the big picture. But the real story is in the weekly details — where symptoms shift and the baby transforms from a cluster of cells to a fully formed infant.

What Are the Pregnancy Week by Week Symptoms?

First trimester symptoms (weeks 1–12)

Nausea typically peaks around week 9 (NHS UK (national health service))
Fatigue common in first trimester (Pregnancy Birth & Baby (Australian Government))
Breast tenderness often first sign (March of Dimes (maternal health organization))
Frequent urination and mood changes reported (Pregnancy Birth & Baby (Australian Government))

Why this matters

A pregnant person facing nausea by week 6 may worry something is wrong. But nausea that peaks around week 9 is a normal hCG-driven response — it signals a pregnancy that’s progressing as expected.

Second trimester changes (weeks 13–27)

Morning sickness eases for most by week 14 (Pregnancy Birth & Baby (Australian Government))
Back pain often starts in second trimester (NHS UK (national health service))
Linea nigra (dark line on belly) appears (March of Dimes (maternal health organization))
Light-headedness or dizziness possible around week 18 (Pregnancy Birth & Baby (Australian Government))

Third trimester signs (weeks 28–40)

Braxton Hicks contractions may begin (NHS UK (national health service))
Breathlessness possible as uterus expands (Pregnancy Birth & Baby (Australian Government))
Sleep disruption common due to discomfort (March of Dimes (maternal health organization))
Lightening (baby dropping) may ease breathing (NHS UK (national health service))

The pattern: first trimester symptoms hit hardest, second trimester often brings a window of relief, and third trimester introduces physical discomfort that signals the body’s final preparations.

How Does Baby Size Change Week by Week?

Baby size comparisons per week

At week 6, embryo is about 1 cm long (March of Dimes (maternal health organization))
At week 12, baby about size of a plum (Pregnancy Birth & Baby (Australian Government))
At week 20, baby about length of a banana (March of Dimes (maternal health organization))
By week 36, baby approximately 6 pounds (NHS UK (national health service))

Growth milestones from conception to birth

Week 5: Neural tube forms; arm/leg buds appear (March of Dimes (maternal health organization))
Week 7: Bones begin forming, eyelids stay shut (March of Dimes (maternal health organization))
Week 12: All organs, muscles, limbs, bones in place (Pregnancy Birth & Baby (Australian Government))
Week