Why does gestational age start counting before conception?

Gestational age is counted from the first day of the last menstrual period (LMP), not from conception. This creates a counterintuitive situation: the first two weeks of 'pregnancy' are technically before the egg was fertilised. Why this system exists: 1. The LMP date is reliably known — women know (or can estimate) when their period started 2. The exact date of ovulation and fertilisation is rarely known with certainty 3. Obstetricians have used LMP-based dating for over 200 years 4. Standardisation: every doctor worldwide uses the same reference point What happens in weeks 1 and 2: - Week 1: your period is occurring - Week 2: follicle developing, oestrogen rising, approaching ovulation - Ovulation occurs around day 14 (end of week 2) - Conception occurs at or shortly after ovulation - Week 3 onwards: the fertilised egg divides and travels to the uterus - Week 4: implantation; hCG detectable; positive pregnancy test possible So when your doctor says you are 6 weeks pregnant, your embryo is actually about 4 weeks old from the point of fertilisation. This is normal and expected — the 6-week measurement is the gestational age, which includes the 2 pre-conception weeks.

How accurate is gestational age calculated from LMP?

Accuracy depends on the regularity of your cycles: For regular 28-day cycles: - LMP-based dating is accurate to ±1-2 weeks - Still not as precise as ultrasound measurement For irregular cycles: - LMP dating can be significantly off if ovulation doesn't occur on day 14 - Example: 35-day cycle — ovulation at day 21, not day 14; LMP dating underestimates age by 7 days - 45-day cycle — could be off by up to 2-3 weeks Why ultrasound is more accurate: - First-trimester ultrasound (before 14 weeks) measures crown-rump length (CRL) - CRL has been correlated to gestational age in large population studies - Accuracy: ±5-7 days in the first trimester - Second and third trimester ultrasound is less accurate for dating (±2-3 weeks) If your ultrasound differs from your LMP dating: - Difference of 7 days or less: clinicians usually keep LMP date - Difference of more than 7 days: EDD is typically revised to match ultrasound - The ultrasound date is always considered more reliable in the first trimester Conclusion: LMP dating gives a good starting estimate. First-trimester ultrasound gives the most accurate gestational age and EDD.

What do the trimester dates mean and when do they start?

Pregnancy is divided into three trimesters, each roughly 13 weeks long: First Trimester (Weeks 1-12): - Starts: Day 1 of LMP - Ends: End of week 12 (day 84 from LMP) - Key features: all major organ systems form; highest risk of miscarriage (80% of all miscarriages); morning sickness most common; embryo becomes a foetus at week 10 - Screening: NT scan (11-13w), first trimester blood tests Second Trimester (Weeks 13-26): - Starts: Day 85 from LMP (week 13, day 0) - Often called the 'honeymoon trimester' — morning sickness usually resolves - Key features: rapid growth; quickening (movement felt 18-20w); anatomy scan at 20w; viability threshold at 24w - Miscarriage risk drops significantly after week 13 Third Trimester (Weeks 27-40+): - Starts: Day 183 from LMP (week 27, day 0) - Key features: rapid weight gain; lung maturation; GBS screening at 36w; full term at 37w - Most births occur weeks 38-42 Note: some sources define T2 as starting at week 14 and T3 at week 28. The boundaries above (13/27) are commonly used but not universally standardised — what matters clinically is the specific week, not which trimester label it falls under.

What is the difference between 'weeks pregnant' and 'months pregnant'?

Pregnancy is officially measured in weeks, not months — because 'months' are ambiguous (different months have different numbers of days) and clinical decisions are made week by week. Approximate month conversions: Month 1: Weeks 1-4 (days 1-28) Month 2: Weeks 5-8 (days 29-56) Month 3: Weeks 9-13 (days 57-91) Month 4: Weeks 14-17 (days 92-119) Month 5: Weeks 18-22 (days 120-154) Month 6: Weeks 23-27 (days 155-189) Month 7: Weeks 28-31 (days 190-217) Month 8: Weeks 32-35 (days 218-245) Month 9: Weeks 36-40 (days 246-280) Why 9 months = 40 weeks (not 36 weeks): If you count 9 calendar months of 4 weeks each = 36 weeks. But pregnancy is 40 weeks because: - Calendar months average 30.4 days (not 28) - 9 × 30.4 = 273.6 days ≈ 39 weeks - With the pre-conception 2 weeks = 40 weeks total Some people say pregnancy is '10 months' — this is approximately correct if using lunar months (4 weeks each): 40 weeks ÷ 4 = 10 lunar months. Bottom line: always use weeks when communicating with healthcare providers. Saying 'I'm 6 months pregnant' is ambiguous. Saying '26 weeks' is precise.

What is the viability threshold and what does it mean?

Viability in obstetrics refers to the gestational age at which a baby can potentially survive outside the womb with medical support. This threshold has changed significantly over decades: Current threshold (most developed countries): 22-24 weeks - Before 22 weeks: survival is extremely unlikely even with maximal support - 22 weeks: survival possible but very rare; outcome highly dependent on the specific hospital and circumstances - 23 weeks: survival rate ~10-30% in specialist centres; high rates of long-term disability in survivors - 24 weeks: survival rate ~50-60%; still significant risk of long-term complications - 25 weeks: survival rate ~70-80% - 28 weeks: survival rate ~90% - 34 weeks: survival rate >99% with mostly short-term NICU care Why it matters: - Before viability: miscarriage and neonatal loss - After viability: premature birth — baby may require NICU care - The 24-week threshold is legally significant in many countries for clinical decision-making This calculator marks 24 weeks as the 'viability threshold' milestone — the point most widely used in clinical guidelines. This does not mean a baby born at 23 weeks cannot survive, nor that outcomes at 24 weeks are guaranteed. Individual circumstances, hospital capabilities, and specific medical factors matter enormously.

How Many Weeks Pregnant Am I?

The Weeks Pregnant Calculator works out your exact gestational age (weeks + days) from today's date using three input methods: Last Menstrual Period (LMP), Due Date (EDD), or Conception Date. Results show gestational age on a colour-coded circular progress ring (green T1, blue T2, orange T3), trimester, days remaining ..

ENTER YOUR DATES

I KNOW MY

FIRST DAY OF LAST MENSTRUAL PERIOD

The first day of actual bleeding of your last period.

Enter a date to calculate your gestational age.

Created with ❤️ byeaglecalculator.com

HOW TO USE

1
Choose your input method — Last Period (LMP date), Due Date (EDD), or Conception Date. All three methods derive the same LMP internally and produce the same gestational age result. LMP is the most common clinical standard. EDD is useful if you have your scan report but not your LMP. Conception date is useful for IVF or tracked ovulation cycles.
2
Enter the relevant date using the date picker. For LMP, this is the first day of actual bleeding of your last period. For EDD, enter the date from your scan or doctor. For conception, enter the estimated date of conception — the calculator adds 14 days internally to get the LMP equivalent.
3
Your gestational age appears immediately as a circular progress ring showing weeks and days, colour-coded by trimester: green for 1st trimester, blue for 2nd, orange for 3rd. The ring fills proportionally from 0% to 100% (40 weeks). Below the ring: your trimester, days remaining until due date, and percentage of pregnancy completed.
4
See your baby's current size compared to a fruit or vegetable — from a poppy seed at 4 weeks to a watermelon at 40 weeks, with approximate length in centimetres and weight in grams. Your most recent completed milestone and your next upcoming milestone are shown below with the milestone's date.
5
Check the right panel for the complete milestone timeline — all 15 key pregnancy events from week 4 (implantation) to week 42 (post-term), each with date and icon. Past milestones are greyed at 50% opacity, the next upcoming is highlighted in black. The week-by-week reference table shows 15 key weeks with their date, trimester, and baby size comparisons.

WORKED EXAMPLE

How Many Weeks Pregnant Am I — worked examples: FROM LMP: LMP: January 1, 2025 Today: April 14, 2025 Days = Apr 14 − Jan 1 = 103 days Weeks = floor(103/7) = 14 weeks Days remainder = 103 mod 7 = 5 days Result: 14 weeks 5 days EDD = Jan 1 + 280 = October 8, 2025 Days remaining = Oct 8 − Apr 14 = 177 days Progress = 103/280 × 100 = 36.8% Trimester: 2nd (weeks 13-26) Baby size: avocado 🥑 (~11.6cm, ~100g) FROM EDD: EDD: October 8, 2025 LMP equivalent = Oct 8 − 280 = January 1, 2025 Then calculate GA as above → same result: 14w 5d FROM CONCEPTION DATE: Conception: January 14, 2025 LMP equivalent = Jan 14 − 14 = January 1, 2025 Then calculate GA as above → same result: 14w 5d MILESTONE DATES (from LMP Jan 1, 2025): Week 13 (end T1): April 2, 2025 ← PASSED Week 18 (quickening): May 7, 2025 Week 20 (anatomy scan): May 21, 2025 Week 24 (viability): June 18, 2025 Week 27 (end T2): July 9, 2025 Week 37 (full term): September 17, 2025 Week 40 (EDD): October 8, 2025

REFERENCE FORMULAS

FORMULA REFERENCE TABLE

Concept	Formula	Description
GA from LMP	GA days = today − LMP; weeks = floor(days/7); remainder = days mod 7	Gestational age is counted in complete weeks + remaining days from the first day of the last period.
GA from EDD	LMP = EDD − 280; then GA from LMP	Work backwards from due date using Naegele's rule (EDD = LMP + 280). Then calculate GA as above.
GA from conception	LMP equivalent = conception − 14; then GA from LMP	Conception assumed to occur 14 days after LMP. Subtract 14 days to get LMP equivalent, then use GA formula.
EDD	EDD = LMP + 280 days	Naegele's rule. 280 days = 40 weeks = 9 calendar months + 7 days.
Days remaining	Days left = EDD − today	If negative, pregnancy is post-term. EDD is an estimate — most births occur within 2 weeks of EDD.
Progress %	% = (GA days / 280) × 100	Percentage of 40-week full-term pregnancy completed. At 20w0d = exactly 50%.
Trimester	T1: wk 1-12 · T2: wk 13-26 · T3: wk 27-40	Trimester boundaries are approximate. ACOG 2013 full term definition: 39w0d–40w6d.

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Last updated: April 25, 2026 · Gestational age calculated from today's date · Baby sizes: ACOG/WHO fetal growth data