How do pregnancy months relate to weeks?

Pregnancy is 40 weeks long but described as 9 months. The months don't map neatly onto calendar months — instead, each pregnancy month covers approximately 4 weeks: Month 1: Weeks 1–4 (1st trimester) Month 2: Weeks 5–8 (1st trimester) Month 3: Weeks 9–12 (1st trimester) Month 4: Weeks 13–16 (2nd trimester) Month 5: Weeks 17–20 (2nd trimester) Month 6: Weeks 21–24 (2nd trimester) Month 7: Weeks 25–28 (3rd trimester) Month 8: Weeks 29–32 (3rd trimester) Month 9: Weeks 33–40 (3rd trimester) Note: month 9 spans 8 weeks (weeks 33-40) because the final trimester extends through to the due date. The 40 weeks of pregnancy divide into exactly 9 obstetric months of 4 weeks each, with month 9 absorbing the 8 final weeks. Why 40 weeks = 9 months: 40 ÷ 4.33 (average weeks per calendar month) ≈ 9.2 months But obstetric months are counted as exactly 4 weeks each: 8 months × 4 weeks = 32 weeks 1 final month × 8 weeks = 40 weeks total This is also why '9 months pregnant' colloquially refers to the entire 40-week journey, even though individual calendar months vary in length.

What are the three trimesters and when do they start?

Pregnancy is divided into three trimesters, each with distinct developmental phases: 1st Trimester — Weeks 1–12 (Months 1–3): - Embryonic development: all major organs form from scratch - Highest vulnerability period (most miscarriages and birth differences occur here) - Heart beating by week 5-6 - Embryo becomes foetus at week 10 - 1st trimester ends at the end of week 12 - NT scan offered at 11-13 weeks 2nd Trimester — Weeks 13–26 (Months 4–6): - The 'honeymoon trimester' — nausea subsides, energy returns - Baby's organs mature and become functional - Quickening (first movements) felt around weeks 18-20 - 20-week anatomy scan - Viability threshold reached at week 24 - Often considered the most comfortable trimester 3rd Trimester — Weeks 27–40 (Months 7–9): - Rapid weight gain — baby gains most of its body fat - Lungs mature in preparation for birth - Brain development continues at high pace - Baby moves into head-down position (usually by week 36) - Full term at week 37 - Due date at week 40 Note: some definitions place the 2nd trimester start at week 14 (rather than 13) and the 3rd trimester start at week 28 (rather than 27). The definitions used here (12/13 and 26/27 boundaries) are the most commonly used in clinical obstetrics.

Is it normal for baby movements to decrease in month 9?

No — it is a common misconception that baby movements decrease in the final weeks because the baby 'runs out of room'. Baby movements should NOT decrease in month 9. What actually happens in month 9: - The pattern of movements may change (less somersaults, more stretches and kicks) - The overall frequency and strength of movements should remain consistent - The baby should still move regularly every day until labour The 'running out of room' myth is dangerous: Decreased fetal movement (DFM) is a recognised warning sign that can indicate: - Fetal distress - Placental insufficiency - Cord compression - In rare cases, serious complications What to do if you notice reduced movement: - Do NOT wait until your next appointment - Contact your midwife or maternity unit immediately (any time, day or night) - You may be offered a CTG (cardiotocograph) to monitor the baby's heart rate - The RCOG and NHS guidelines: 'If you think your baby has reduced its movements, contact your midwife or maternity unit immediately' Kick counting: From week 28, try to be aware of your baby's normal movement pattern. There is no set number of movements that is 'normal' — it varies by baby. What matters is a change from your baby's established pattern. If in doubt, always call.

What is the difference between full term (week 37) and due date (week 40)?

These are two distinct milestones that are often confused: Full term — Week 37: - Defined as 37 weeks 0 days of gestation - Baby is considered 'ready' — all organs capable of functioning independently - Labour starting at or after 37 weeks is not considered premature - A baby born at week 37 will usually not need NICU care for prematurity - However, 37-week babies may still face more challenges than 39-40 week babies Due date — Week 40 (EDD): - The Estimated Due Date (EDD) is set at 40 weeks from LMP - Based on Naegele's rule: LMP + 280 days - Only ~5% of babies are born on their exact due date - ~80% of babies are born between weeks 38 and 42 Early term (37-38w) vs Full term (39-40w): Research shows that 39-40 week babies on average have: - Better lung maturity - Better feeding outcomes - Lower NICU admission rates - Slightly higher birth weight - Better long-term neurodevelopmental outcomes This is why elective induction before 39 weeks is generally avoided unless medically indicated, even though the baby is technically 'full term' at 37 weeks. Post-term (41-42w): - After 41 weeks, the placenta may begin to age and function less efficiently - Induction is typically offered at 41-42 weeks - The risks of continuing the pregnancy beyond 42 weeks begin to outweigh the risks of induction

Pregnancy Month-by-Month Calculator — All 9 Months

The Pregnancy Month-by-Month Calculator shows baby development, size, symptoms and appointments for every month of pregnancy. Select any of the 9 months to see: baby size at start and end of month (with emoji, size name, length in cm, weight in grams or kg), week-by-week developmental highlights for..

SELECT YOUR MONTH

MONTH 5 · WEEKS 17–20 · 2ND TRIMESTER

START

🍐

pear

of 9 months

40–50%

END

🍌

banana

16cm · 300g

WEEK-BY-WEEK DEVELOPMENT

Week 17: Fat stores developing. Baby can hear sounds outside the womb.

Week 18: Quickening (first movements) felt. 20-week scan approaching.

Week 19: Vernix caseosa coating skin. Brain designating sensory areas.

Week 20: Halfway point! Anatomy scan. Baby swallowing amniotic fluid.

🌸 HOW YOU MAY FEEL

Quickening — first baby movements felt (like flutters). Halfway milestone at week 20. Anatomy scan scheduled.

COMMON SYMPTOMS — MONTH 5

Baby movements (quickening)Back pain beginningLeg crampsSkin changesHeartburnStuffy nose

APPOINTMENTS & TESTS — MONTH 5

📋

20-week anatomy scan (detailed fetal anomaly scan)

📋

20-week antenatal check

💡 MONTH 5 TIP

The 20-week anatomy scan checks organ development, placenta position and amniotic fluid. Gender can usually be confirmed.

ALL 9 MONTHS AT A GLANCE

Mo	Weeks	T	End size	Weight
1	1–4	T1	🌸 poppy seed	tiny
2	5–8	T1	🍓 raspberry	1g
3	9–12	T1	🍋 lime	14g
4	13–16	T2	🥑 avocado	100g
5	17–20	T2	🍌 banana	300g
6	21–24	T2	🌽 ear of corn	600g
7	25–28	T3	🍆 eggplant	1.00 kg
8	29–32	T3	🍍 pineapple	1.70 kg
9	33–40	T3	🍉 watermelon	3.46 kg

Created with ❤️ byeaglecalculator.com

HOW TO USE

1
Click any of the 9 month buttons in the selector grid — colour-coded by trimester (green T1 months 1-3, blue T2 months 4-6, orange T3 months 7-9). The button shows the month number, week range, and trimester label. The entire page updates instantly.
2
Read the hero panel: three columns showing baby size at the start of the month (emoji + size name), the month number in large coloured type with a progress bar segment, and baby size at the end of the month (emoji + name + cm length + weight in grams or kg).
3
Review the week-by-week development section — 4 rows covering each week of the selected month (month 9 covers 8 weeks). Each row shows the week number and the key developmental milestone for that week. Below: a yellow 'How you may feel' card with typical maternal symptoms.
4
Check the symptoms chip panel (coloured pill badges for all common symptoms that month), the appointments & tests checklist (what to expect at the doctor or midwife), and the green tip card with a high-value clinical tip specific to that month.
5
Use the all-9-months reference table at the bottom right to compare all months at a glance — showing week range, trimester, end-of-month baby size, and weight. Click any row to jump to that month.

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Last updated: April 25, 2026 · All 9 months of pregnancy · Sources: ACOG, NHS, WHO