What is gestational age and how is it counted?

Gestational age (GA) is the official medical measurement of how far along a pregnancy is. It is counted in weeks and days from the first day of the last menstrual period (LMP) — not from the date of conception. This creates a counterintuitive starting point: - Week 1 and Week 2: your period and the lead-up to ovulation — no pregnancy yet - Week 3: fertilisation occurs — conception happens at the end of week 2 or during week 3 - Week 4: implantation complete — hCG detectable, positive test possible - Week 5+: embryonic development proper begins Why count from LMP and not conception? 1. The LMP date is reliably known; conception date is rarely known precisely 2. Ovulation/fertilisation timing varies between individuals 3. This system has been used in obstetrics for over 200 years worldwide 4. Standardisation: all healthcare providers use the same reference point Result: 'gestational age' always runs approximately 2 weeks ahead of 'embryonic age' (time since conception). When your doctor says 'you are 10 weeks pregnant,' your baby's embryo is actually about 8 weeks old from fertilisation. GA format: expressed as 'weeks + days' — e.g. 14w3d = 14 weeks and 3 days from LMP = 101 days total.

What happens in weeks 1, 2, and 3 of pregnancy?

Weeks 1-3 are the earliest part of the gestational calendar — before most women know they are pregnant: Week 1 (Days 1-7 from LMP): - Your period is occurring - No embryo exists yet - The uterine lining sheds and rebuilds - Your body is preparing for ovulation - Counted as 'week 1 of pregnancy' only by convention Week 2 (Days 8-14 from LMP): - A follicle matures in one ovary - Oestrogen rises, stimulating uterine lining growth - Ovulation occurs around day 14 (end of week 2 for a 28-day cycle) - The egg is released — fertile window open - Sperm can fertilise the egg for approximately 12-24 hours after ovulation Week 3 (Days 15-21 from LMP): - Fertilisation may occur (sperm meets egg in fallopian tube) - A zygote forms and immediately begins dividing - The fertilised egg travels down the fallopian tube over 3-4 days - By end of week 3, the blastocyst reaches the uterus - Implantation begins (6-10 days after fertilisation) No symptoms: in weeks 1-3, there are typically no pregnancy symptoms because hCG has not yet been produced in detectable quantities. The first positive home pregnancy test is usually possible at week 4, when hCG levels are high enough to detect. IVF note: in IVF cycles, the embryo's age is counted from egg retrieval (fertilisation day), which is equivalent to approximately day 14 of a natural cycle. So a day-5 blastocyst transfer gives an 'LMP equivalent' of approximately 19 days before the transfer date.

What is the difference between a foetus and an embryo?

The transition from embryo to foetus occurs at the end of week 10 / beginning of week 11 of gestation: Embryonic period (Weeks 3-10): - From fertilisation through week 10 - The embryo forms all major organ systems from scratch - This is the most critical period for development - Highest vulnerability to teratogens (substances that can cause birth defects) - By the end of week 8, the embryo has all basic body structures - The embryo grows from a few cells to ~3cm Foetal period (Weeks 11-40): - From week 11 onwards, the embryo is called a foetus - The term shift reflects that all major organs have been formed - The foetus grows, matures, and refines existing structures - Organs become functional (kidneys produce urine by week 12, brain waves detectable by week 17) - The foetus gains most of its weight in the third trimester - Fat accumulates from week 17 onwards Key embryonic milestones: - Week 4: heart begins beating - Week 5: neural tube closes (brain and spinal cord forming) - Week 6: arm and leg buds appear - Week 8: all major organs present in rudimentary form; fingers forming - Week 10: embryonic period ends; now called a foetus Why does the terminology matter? - The embryonic period is when most major congenital differences arise - Folic acid is most critical before and during weeks 3-6 (neural tube formation) - After week 10, the risk of major structural abnormalities from most teratogens decreases significantly

When can I find out the baby's sex?

The baby's biological sex (chromosomal sex) is determined at fertilisation but is not detectable until the genitals differentiate: Chromosomal testing (earliest — from week 9-10): - NIPT (Non-Invasive Prenatal Testing): blood test from 9-10 weeks detects fetal DNA in maternal blood - Can determine chromosomal sex from week 9-10 - Also screens for chromosomal conditions (Down syndrome, trisomy 18, 13) - Not 100% accurate — ~99% sensitivity for sex determination CVS (Chorionic Villus Sampling) from week 10-13: - Diagnostic test (not screening) — highly accurate - Invasive procedure, usually only offered when there is clinical indication - Can determine sex and diagnose chromosomal conditions Ultrasound (most common — from week 16-20): - External genitalia visible on ultrasound from ~week 16-18 - Most accurate at 18-20 weeks (anatomy scan) - Accuracy: ~99% at 18 weeks when baby is positioned cooperatively - Baby's position can obstruct a clear view Amniocentesis (diagnostic, from week 15-20): - Highly accurate chromosomal analysis - Invasive — usually offered only for clinical indications Common misconception: the baby's genitals don't 'form' during pregnancy — the chromosomal sex is set at fertilisation. What develops during pregnancy is the physical expression of that chromosomal sex, which becomes visible on ultrasound around weeks 16-20.

What are Braxton Hicks contractions and when do they start?

Braxton Hicks contractions are irregular, painless uterine contractions that occur throughout pregnancy — but become noticeable usually from around week 20-22 in first pregnancies and sometimes earlier in subsequent pregnancies. What they are: - The uterine muscle (myometrium) contracts sporadically - Known as 'practice contractions' or 'false labour' - First described by John Braxton Hicks in 1872 - Serve to tone the uterine muscle and may increase blood flow to the placenta Characteristics of Braxton Hicks: - Irregular — no regular pattern or timing - Unpredictable — come and go without progression - Usually painless or mildly uncomfortable — not intense - Typically last 30 seconds to 2 minutes - Often triggered by dehydration, physical activity, sex, or full bladder - Usually subside with rest, movement, or hydration - Do not cause cervical dilation When to be concerned (real labour vs Braxton Hicks): - If contractions become regular (every 5-10 minutes) before week 37 - If contractions are intensifying and not going away - If accompanied by bleeding, fluid leaking, or severe pain - Before 37 weeks: may indicate preterm labour — call your doctor immediately Before week 37: all regular contractions should be reported to your healthcare provider, as preterm labour is a medical emergency. After week 37 (full term), regular contractions may be early labour — follow your birth plan and contact your midwife or hospital.

Pregnancy Week-by-Week Calculator — Weeks 1 to 40

The Pregnancy Week-by-Week Calculator shows baby development, size, and what to expect at every week from week 1 (pre-conception LMP week) through week 40 (due date). Enter your gestational age in weeks and optional days. Results show: baby size comparison with emoji (poppy seed to watermelon), crown-to-rump length in ..

HOW MANY WEEKS PREGNANT?

WEEKS

wks

ADDITIONAL DAYS (0-6)

days

PREGNANCY WEEK 20

🍌

BABY SIZE

banana

16 cm · 300 g

TRIMESTER

2nd

COMPLETE

50%

REMAINING

20w 0d

👶 BABY THIS WEEK

Halfway point! Anatomy scan this week. Baby swallowing amniotic fluid.

🌸 HOW YOU MAY FEEL

20-week anatomy scan. Halfway milestone. Gender reveal possible.

PREGNANCY PROGRESS

T1 (1-12)T2 (13-26)T3 (27-40)

KEY MILESTONES

✓

Wk 6 — Heartbeat detectable

✓

Wk 12 — 1st trimester ends

✓

Wk 13 — 2nd trimester begins

📷

Wk 20 — Anatomy scan

← now

🏥

Wk 24 — Viability threshold

in 4w

🌟

Wk 27 — 3rd trimester begins

in 7w

🧪

Wk 36 — Group B Strep test

in 16w

✅

Wk 37 — Full term — safe to deliver

in 17w

🎂

Wk 40 — Due date (EDD)

in 20w

JUMP TO ANY WEEK

T1 TRIMESTER

T2 TRIMESTER

T3 TRIMESTER

Created with ❤️ byeaglecalculator.com

HOW TO USE

1
Enter your current gestational age in the WEEKS field — the number of complete weeks pregnant. Use the number pad or the quick-select buttons for key milestones (week 4, 12, 20, 27, 37, 40). The calculator covers weeks 1-40, including weeks 1-2 (pre-conception period weeks counted from LMP) and week 3 (fertilisation and conception).
2
Enter any additional days (0-6) in the DAYS field for precision. For example, if you are 14 weeks and 3 days pregnant, enter 14 weeks + 3 days. The seven day buttons (0d-6d) let you select quickly. This gives an exact gestational age of 14w 3d.
3
Read your results: baby's current size comparison (with emoji, name, crown-to-rump length in cm, and weight in grams or kg), trimester, percentage of pregnancy completed, and weeks + days remaining until 40 weeks. The dark result card shows baby development highlights and how you may feel this week.
4
Check the circular progress ring on the right — colour-coded by trimester (green T1, blue T2, orange T3) — and the trimester bar below it showing your exact position across all 40 weeks. The milestone tracker shows 9 key events from heartbeat to due date, with past milestones greyed out, the current week highlighted, and upcoming milestones showing how many weeks away they are.
5
Use the week navigator chips to jump directly to any week across all three trimesters. Click any row in the reference table to jump to that week — it shows all 40 weeks with trimester, emoji, size name, length, weight, and a short development note. The current week is highlighted in black.

REFERENCE — ALL 40 WEEKS AT A GLANCE

WEEK-BY-WEEK REFERENCE TABLE (WEEKS 1–40)

Week	Trimester	Baby	Size	Length	Weight	Development
1	T1	🔬	microscopic	—	—	Period week — no conception yet. Uterine lining shedding. Ge...
2	T1	🔬	microscopic	—	—	Follicle maturing in ovary. Oestrogen rising. Ovulation appr...
3	T1	📍	pin head	0.05 cm	—	Fertilisation and conception occurs. Zygote forms and begins...
4	T1	🌸	poppy seed	0.10 cm	—	Implantation complete. hCG detectable — positive test possib...
5	T1	🌱	sesame seed	0.20 cm	—	Heart begins beating. Neural tube forming. Amniotic sac deve...
6	T1	🫘	lentil	0.50 cm	—	Heartbeat detectable by transvaginal ultrasound (~100-160 bp...
7	T1	🫐	blueberry	1 cm	—	Brain developing rapidly. Arm and leg buds visible. Face for...
8	T1	🍓	raspberry	1.6 cm	1 g	All major organs forming. Fingers beginning to form. Tail di...
9	T1	🍇	grape	2.3 cm	2 g	Officially a foetus. Eyelids fused. Tongue forming. Muscles ...
10	T1	🍊	kumquat	3.1 cm	4 g	Embryonic period ends. Vital organs functional. Fingernails ...
11	T1	🟤	fig	4.1 cm	7 g	Genitals forming. Baby can open and close fists. Tooth buds ...
12	T1	🍋	lime	5.4 cm	14 g	1st trimester nearly complete. Reflexes developing. Kidneys ...
13	T2	🫛	pea pod	7.4 cm	23 g	2nd trimester begins. Fingerprints forming. Vocal cords deve...
14	T2	🍋	lemon	8.7 cm	43 g	Neck longer. Baby making facial expressions. Lanugo (fine ha...
15	T2	🍎	apple	10.1 cm	70 g	Taste buds forming. Skeletal system developing. Baby may suc...
16	T2	🥑	avocado	11.6 cm	100 g	Gender may be visible on ultrasound. Eyes making small movem...
17	T2	🍐	pear	13 cm	140 g	Fat stores developing. Baby can hear sounds outside the womb...
18	T2	🫑	bell pepper	14.3 cm	190 g	Quickening likely (first movements felt). 20-week scan appro...
19	T2	🥭	mango	15.2 cm	240 g	Vernix caseosa coating skin. Brain designating sensory areas...
20	T2	🍌	banana	16 cm	300 g	Halfway point! Anatomy scan this week. Baby swallowing amnio...
21	T2	🥕	carrot	26.7 cm	360 g	Eyebrows and eyelids fully formed. Rapid brain development.
22	T2	🥦	spaghetti squash	27.8 cm	430 g	Grip reflex developing. Lips more distinct. Sleep cycles est...
23	T2	🍊	grapefruit	28.9 cm	501 g	Hearing more acute — responds to familiar voices. Lungs deve...
24	T2	🌽	ear of corn	30 cm	600 g	Viability threshold — could survive with intensive NICU care...
25	T2	🥦	cauliflower	34.5 cm	660 g	Hands fully formed. Baby responds to touch on belly. Fat acc...
26	T2	🥬	lettuce head	36.8 cm	760 g	Eyes open for first time. Brain growing rapidly. Immune syst...
27	T3	🥬	bunch of kale	38 cm	875 g	3rd trimester begins. Lungs maturing. Baby can hiccup.
28	T3	🍆	eggplant	39 cm	1.00 kg	REM sleep begins — baby may be dreaming. Lanugo disappearing...
29	T3	🎃	butternut squash	39.7 cm	1.15 kg	Bones hardening. Muscle tone improving. Head may engage in p...
30	T3	🥬	cabbage	40 cm	1.32 kg	Red blood cells producing in bone marrow. Toenails visible.
31	T3	🥥	coconut	41.2 cm	1.50 kg	All 5 senses functioning. Rapidly gaining weight — 250g/week...
32	T3	🍍	pineapple	42.4 cm	1.70 kg	Practice breathing movements. Fingernails reach fingertips.
33	T3	🍈	honeydew melon	43.6 cm	1.92 kg	Skull bones not yet fused (for delivery). Immune system boos...
34	T3	🍈	cantaloupe	44.9 cm	2.15 kg	Central nervous system and lungs maturing. Most babies head-...
35	T3	🥬	romaine lettuce	46.1 cm	2.38 kg	Kidneys fully developed. Liver processing waste. Nearly full...
36	T3	🍈	papaya	47.3 cm	2.62 kg	Most babies head-down. Group B Strep test this week.
37	T3	🍉	winter melon	48.5 cm	2.86 kg	Full term! Safe to deliver. Baby has firm grasp. All organs ...
38	T3	🧅	leek	49.7 cm	3.08 kg	Gaining ~30g/day. Vernix thickening. Baby preparing for birt...
39	T3	🎃	pumpkin	50.8 cm	3.29 kg	Considered full term. Placenta still providing antibodies.
40	T3	🍉	watermelon	51.8 cm	3.46 kg	Due date week! Average birth weight ~3.4kg. Ready for the wo...

Click any row to jump to that week · Length = crown to rump · Sources: ACOG, WHO fetal growth data

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Last updated: April 25, 2026 · Covers weeks 1-40 including pre-conception weeks 1-3 · Baby sizes: ACOG/WHO fetal growth data