This article is for general educational purposes only. Fetal development varies between pregnancies. Consult your obstetrician or midwife for personalised prenatal care.
Before you begin: knowing your due date
Pregnancy weeks are conventionally measured from the first day of your last menstrual period (LMP), not from the date of conception. This means weeks 1–2 happen before ovulation and fertilisation. Use the Pregnancy Due Date Calculator to calculate your estimated due date, fertile window, and the 3 upcoming cycle dates from your LMP and cycle length.
First trimester: weeks 1–13
Weeks 1–2: before conception
These weeks count from LMP but precede fertilisation. The uterine lining builds up, and ovulation occurs near the end of week 2 (around day 14 in a 28-day cycle). Fertilisation, if it occurs, happens within 12–24 hours of ovulation.
Week 3: fertilisation and implantation
The fertilised egg (zygote) travels down the fallopian tube, dividing into a cluster of cells (morula, then blastocyst). It reaches the uterus around day 5–6 after fertilisation and implants in the uterine lining at the end of week 3 or beginning of week 4.
Week 4: pregnancy confirmed
A home pregnancy test can now detect the rising level of human chorionic gonadotropin (hCG). The embryo is about 2 mm — barely visible. The amniotic sac and yolk sac are forming. Many women notice no symptoms yet; others experience implantation bleeding.
Week 5: first symptoms
hCG is rising rapidly. Morning sickness (nausea, sometimes vomiting) often begins this week. The embryonic disk — the precursor to the entire body — is forming three layers (ectoderm, mesoderm, endoderm) that will become the nervous system, muscles, and internal organs.
Week 6: heartbeat detected
The heart begins beating — visible on transvaginal ultrasound at approximately 100–120 bpm. The neural tube closes. Arm and leg buds appear. The embryo is about 6 mm long (the size of a pea).
Weeks 7–8: rapid organogenesis
The brain is growing rapidly. Fingers and toes begin forming (webbed at first, then separating). Eyes, ears, and nose take shape. All major organs are present in rudimentary form by week 8. The embryo is now about 1.6 cm and weighs less than 1 g.
Week 9: embryo becomes fetus
At week 9, the term “embryo” transitions to fetus. Kidneys begin producing urine. The fetus makes small spontaneous movements. Heartbeat is now 140–170 bpm. Nuchal translucency ultrasound is typically offered between weeks 11–13.
Week 10: genitals developing
External genitalia are present but not yet distinguishable. The placenta is taking over hormone production from the corpus luteum. First-trimester screening blood tests (PAPP-A, free β-hCG) are often drawn around this time.
Week 12: end of embryonic organ formation
All major body structures are present. Fingernails start forming. The fetus is approximately 6 cm long and weighs about 14 g. The risk of miscarriage drops significantly after 12 weeks, which is why many couples share pregnancy news now.
Week 13: end of first trimester
The fetus can make facial expressions, swallow amniotic fluid, and move its arms and legs. The placenta is fully functional. The uterus begins to rise above the pubic bone.
Second trimester: weeks 14–27
Weeks 14–16: “the golden trimester”
Morning sickness typically eases. Energy returns. The fetus begins making sucking movements. Fine hair (lanugo) covers the body for warmth. At week 15–16, amniocentesis (if chosen) can be performed.
Week 18–20: anatomy scan
The detailed 20-week anomaly scan checks all major organs, placenta position, amniotic fluid levels, and fetal growth. Fetal sex can often be determined. The fetus weighs about 300 g and is around 25 cm crown-to-rump.
Week 20: quickening
Most women feel the first fetal movements (“quickening”) between weeks 16–25. First-time mothers often feel it later than experienced ones. Movements feel like flutters or light taps.
Week 24: viability threshold
24 weeks is conventionally considered the threshold of viability — the point at which a premature baby born at this stage has a chance of survival with intensive neonatal care (survival rates at 24 weeks are 40–70% at specialist centres). Surfactant production in the lungs begins.
Week 25–27: rapid weight gain
The fetus gains approximately 100 g per week. Brain activity increases. The fetus responds to sound — can startle at loud noises. Eyes begin opening and closing. Glucose challenge testing (GCT) for gestational diabetes is typically done between weeks 24–28.
Third trimester: weeks 28–40
Week 28: third trimester begins
All five senses are functional. The fetus practices breathing (using amniotic fluid). Brain surface begins forming folds (gyri and sulci). Weight is approximately 1 kg.
Week 32: lung maturation
Surfactant production accelerates. Babies born at 32 weeks have excellent survival rates (>95%) and typically spend 4–6 weeks in the NICU. The fetus is approximately 1.8 kg.
Week 34–36: preparation for birth
The fetus moves into a head-down position (cephalic presentation) for most pregnancies. Lanugo begins shedding. White vernix (protective coating) is still present. Growth rate slows. Weight: approximately 2.5–2.8 kg.
Week 37: early term
From 37 weeks, the pregnancy is “early term.” Lungs are considered mature. However, studies show that babies born at 37–38 weeks have higher rates of certain complications (feeding difficulties, respiratory issues) than those born at 39+ weeks. Unless medically indicated, delivery before 39 weeks is generally avoided.
Week 39–40: full term
Full term begins at 39 weeks. The cervix begins softening (effacing) and may start to dilate. Braxton Hicks contractions become more regular. The baby is now 50–52 cm long and weighs on average 3.3–3.5 kg, though there is wide normal variation.
Week 41–42: post-term
If labour has not begun spontaneously by week 41, most providers will discuss labour induction due to increasing risks (placental insufficiency, meconium in amniotic fluid, increased stillbirth risk after 42 weeks). Most women deliver by 42 weeks.
Tracking your pregnancy timeline
Calculate your estimated due date, fertilisation date, and trimester transitions with the Pregnancy Due Date Calculator. Input your LMP and typical cycle length, and it will give you:
- Estimated due date (Naegele’s Rule)
- Estimated ovulation / fertilisation date
- End of each trimester
- Three upcoming predicted cycle dates
Remember that only 5% of babies are born on their exact due date — most arrive within 2 weeks either side. The due date is a midpoint, not a deadline.