The Three Trimesters
Pregnancy is divided into three trimesters, each with distinct stages of development for your baby and unique changes for your body. Here's a concise overview of what to expect.
Weeks 1–13
Rapid cell division and organ formation. Heartbeat detectable around 6 weeks. Embryo becomes a fetus at week 10. Common symptoms: nausea, fatigue, breast tenderness, frequent urination.
Weeks 14–27
Often called the "golden trimester." Baby grows from 3.5 inches (lemon) to 14 inches (eggplant). First movements (quickening) around weeks 18–22. Anatomy scan at approximately 20 weeks.
Weeks 28–40
Baby gains most of its weight. Brain development accelerates. Baby typically positions head-down around week 36. Time to prepare for labor, birth, and the fourth trimester.
Key Milestones by Week
A snapshot of the most significant developments at each major stage of pregnancy.
| Week | Size | Development |
|---|---|---|
| 6 | Sweet pea | Heartbeat detectable |
| 8 | Raspberry | Fingers forming, major organs present |
| 12 | Lime | End of highest-risk period, 12-week scan |
| 16 | Avocado | Can hear sounds, gender may be visible |
| 20 | Banana | Anatomy scan, halfway point |
| 24 | Corn | Viability threshold (lungs developing) |
| 28 | Eggplant | Third trimester begins, baby can open eyes |
| 32 | Squash | Most organ systems mature |
| 36 | Honeydew | Baby likely head-down, full term soon |
| 40 | Watermelon | Due date! Baby ready for birth |
Pregnancy questions beyond the weekly guide
Every week page answers "what's happening this week." These articles answer the questions that don't fit neatly into a single week.
Each Trimester — What to Expect
Symptoms, development, and key decisions at every stage.
The rapid-formation period
In 13 weeks, a single fertilised cell becomes a fetus with a beating heart, functioning kidneys, and all major organ systems laid down. This is the highest-risk trimester for miscarriage (most occurring before week 10) and the period when folic acid is most critical.
- Week 4: Embryo implants, size of a poppy seed
- Week 5: Heart tube beats for the first time (~100bpm)
- Week 6: Facial features forming; heartbeat on scan
- Week 8: Fingers and toes present; all organs forming
- Week 10: Embryo becomes a fetus; tail disappears
- Week 12: 12-week scan; nuchal translucency screening
- Week 13: End of highest-risk period; risk of miscarriage drops sharply
- Nausea and vomiting (affects 70–80%; peaks weeks 8–10)
- Extreme fatigue from high progesterone
- Breast tenderness and enlargement
- Frequent urination as kidneys filter 50% more blood
- Bloating and constipation from progesterone
- Heightened sense of smell
- Light spotting (25% of pregnancies; usually harmless)
The "golden trimester"
For most women, nausea resolves and energy returns around week 14. The baby grows from a lemon (3.5 inches) to the size of a head of lettuce (14 inches). You'll likely feel the first movements — called quickening — between weeks 18–22. The anatomy scan at week 20 is the most detailed look at fetal development and structure.
- Week 16: Can hear sounds; hair beginning to grow
- Week 18–22: Quickening — first fetal movements felt
- Week 20: Anatomy scan (20-week ultrasound)
- Week 22: Considered viable (though extremely preterm)
- Week 24: Lungs producing surfactant; viability improves significantly
- Week 27: Eyes open; responds to light and sound
- Nausea resolves for most women by week 14–16
- Baby bump becomes visible (earlier with subsequent pregnancies)
- Round ligament pain — sharp pains when moving quickly
- Nasal congestion and nosebleeds from increased blood volume
- Skin changes: linea nigra, melasma, or stretch marks
- Braxton Hicks contractions may begin (irregular, painless)
- Back pain as centre of gravity shifts
The final stretch
The final 12 weeks are dominated by baby weight gain (the majority of birth weight is gained in the third trimester), brain development, and preparation for birth. Your body is simultaneously preparing for labour — cervical softening, lightening (baby dropping) — and becoming increasingly uncomfortable as space runs out.
- Week 28: Eyelids can open and close; brain grows rapidly
- Week 32: Breathing practice begins; most organ systems mature
- Week 34: Fingernails and toenails fully formed
- Week 36: Typically head-down; "early term" begins
- Week 37: "Full term" — lungs and organs ready for birth
- Week 40: Due date (only 5% of babies born on due date)
- Heartburn as the uterus compresses the stomach
- Shortness of breath as the uterus pushes up into the diaphragm
- Swollen ankles and feet (oedema) — worse at end of day
- Frequent urination returns as baby drops
- Insomnia and difficulty finding a comfortable sleep position
- Braxton Hicks more frequent
- Pelvic pressure ("lightning crotch")
Prenatal Nutrition: What You Actually Need
Based on ACOG and WHO recommendations. No supplement replaces a varied diet — but these six are evidence-backed and worth knowing.
Alcohol (no safe level established), high-mercury fish, raw or undercooked meat and eggs, unpasteurised cheeses and deli meats, excess vitamin A from liver or supplements, caffeine over 200mg/day (~1 standard coffee). Sushi from reputable restaurants is generally considered acceptable if fish is fresh-frozen first.
Symptom Reference: Normal vs. Call Your Provider
Most pregnancy symptoms are normal — but some warrant same-day or urgent contact. This is a general reference, not a substitute for your provider's advice.
| Symptom | Normal | Call your provider if... |
|---|---|---|
| Nausea / vomiting | Weeks 5–14; most intense 8–10 | Unable to keep any fluids down for 24h; losing weight; signs of dehydration |
| Spotting | Light pink/brown in weeks 4–6 (implantation) | Bright red bleeding, heavy flow, clots, or spotting after week 12 |
| Cramping | Mild, intermittent in first and third trimester | Severe, one-sided pain (ectopic risk); regular contractions before week 37 |
| Headaches | Common, especially trimester 1–2 | Severe headache with visual disturbances, swelling, high blood pressure (preeclampsia signs) |
| Swelling | Feet and ankles, especially in third trimester | Sudden facial swelling, hands, or swelling with headache/vision changes |
| Fetal movement | Quickening weeks 18–22; regular by week 28 | After week 28: fewer than 10 movements in 2 hours; significant decrease from baseline |
| Discharge | Clear/white thin discharge throughout pregnancy | Yellow, green, or foul-smelling; watery gush (may be amniotic fluid) |
| Back pain | Lower back ache, especially second and third trimester | Severe back pain with fever (kidney infection); regular rhythmic back pain (labour) |
Remarkable Fetal Development Facts
The neural tube — which becomes the entire brain and spinal cord — forms entirely within the first 4 weeks of pregnancy, often before the woman knows she is pregnant. This is why folic acid supplementation before conception is so critical: once the tube is formed, folic acid can no longer prevent neural tube defects. 70% of these defects are preventable with adequate folic acid.
The cardiac tube begins beating around day 22 post-conception (approximately week 5 by LMP). It's not yet a four-chambered heart — that completes around week 10. By week 5, it beats at 80–100 beats per minute, increasing to 150–175 bpm by week 9, then gradually slowing toward birth.
By week 16, the inner ear is structurally complete and the fetus begins responding to sound. Studies (Kisilevsky et al., 2003) show fetuses recognise their mother's voice by week 33 and respond to familiar music. Reading and talking to your baby in the third trimester has measurable neurodevelopmental effects after birth.
Week 24 is the internationally recognised threshold of viability — the point at which survival outside the womb becomes possible with intensive neonatal care. The critical limiting factor is lung maturity: the lungs begin producing surfactant (which prevents alveolar collapse) at around week 24, with production accelerating through weeks 28–36.
The fetal brain triples in weight between weeks 28 and 40. The characteristic folds and grooves of the brain (sulci and gyri) form primarily in weeks 26–34. Brain development continues rapidly after birth — the brain isn't fully mature until the mid-twenties — but the third trimester is the most critical window for the foundational architecture.
Key Prenatal Appointments & Tests
First prenatal visit. Blood tests (blood type, rubella, STIs, full blood count), blood pressure baseline, due date calculation, referrals for scans.
Confirms gestational age, checks for chromosomal conditions (Down syndrome, trisomy 18/13). Combined with blood tests (PAPP-A, hCG) for screening accuracy.
The most detailed scan of fetal structure. Checks heart, brain, spine, kidneys, lips, limb bones. Also the point at which placenta location and amniotic fluid are assessed. Gender may be shared if requested.
Gestational diabetes affects 6–9% of pregnancies. The 1-hour glucose challenge test (or 2-hour OGTT) identifies blood sugar dysregulation that requires dietary management or medication.
GBS is carried by 25% of women without causing illness. If positive, IV antibiotics are given in labour to prevent transmission to the baby during birth.
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Frequently Asked Questions
When does morning sickness start and end? +
Nausea typically starts between weeks 5–6, peaks at weeks 8–10 when hCG is highest, and resolves for most women by week 14. About 10% of women have nausea beyond the first trimester. 1–2% develop hyperemesis gravidarum (severe vomiting requiring medical treatment).
When will I feel my baby move? +
First-time mothers typically feel "quickening" — the first fetal movements — between weeks 18–22. Women who have been pregnant before often feel it earlier (weeks 16–18) as they recognise the sensation. By week 28, fetal movement should be regular and trackable. A significant decrease in movement after week 28 warrants same-day contact with your provider.
What is the anatomy scan and what does it check? +
The anatomy scan (performed at approximately week 20) is the most detailed ultrasound of the pregnancy. It systematically checks fetal structures: heart (chambers and valves), brain (ventricles), spine, face (cleft lip), abdominal wall, limb bones, kidneys, and placenta position. It also assesses amniotic fluid volume. Gender can be shared at this scan if you want to know.
When is a baby considered full term? +
The ACOG defines full term as weeks 39–40 (previously week 37 was considered full term, but this was revised in 2013). "Early term" is weeks 37–38; babies born at this stage have higher rates of NICU admission and respiratory problems than those born at 39–40 weeks. "Late term" is weeks 41–41+6; "post-term" is 42+ weeks.
Is it safe to exercise during pregnancy? +
Yes — ACOG recommends 150 minutes per week of moderate-intensity exercise for women with uncomplicated pregnancies. Walking, swimming, cycling, prenatal yoga, and strength training (with modifications) are all considered safe. Avoid contact sports, activities with fall risk (skiing, horse riding), hot yoga, and lying flat on your back after week 20. Exercise in pregnancy is associated with lower rates of gestational diabetes, preeclampsia, and caesarean delivery.
What weight gain is normal during pregnancy? +
Institute of Medicine guidelines (used by ACOG): Underweight (BMI <18.5): 28–40 lbs | Normal weight (18.5–24.9): 25–35 lbs | Overweight (25–29.9): 15–25 lbs | Obese (BMI ≥30): 11–20 lbs. These are guidelines — your provider will monitor your individual trajectory. Most women gain very little in the first trimester (1–5 lbs) with the majority of gain occurring in the second and third.
Frequently Asked Questions
Your Postpartum Recovery Guide — the fourth trimester
Pregnancy ends at birth, but your body's recovery takes months. Physical healing, mental health (baby blues vs PPD), pelvic floor, breastfeeding, and the first 12 months — all in BabyBloom's free postpartum guide.
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