π Gestational Age Calculator
Weeks and days, from any dating method.
For informational purposes only, not medical advice. Consult a healthcare professional.
Gestational age counts from the first day of the last period, about two weeks before conception: an LMP of January 1 means roughly 15w 1d on April 17 and a due date of October 8. This calculator dates pregnancy from LMP, conception, ultrasound CRL, or IVF milestones. Pick your method above.
How to Use This Calculator
Obstetrics dates everything from gestational age, so getting it right anchors every screening window and growth chart. Choose the method matching your information.
Pick the dating method. LMP works with regular cycles; ultrasound CRL is the gold standard when cycles are irregular or dates uncertain.
Enter the date (or CRL in mm for ultrasound). For IVF, the transfer or retrieval date converts exactly.
Read weeks+days, the trimester, and the estimated due date. Only 4-5% of babies arrive on the due date; 37-42 weeks is the full-term window.
The Dating Methods
The two-week fiction (counting from LMP before ovulation) exists because ovulation day was historically unknowable while LMP was memorable; the convention stuck because every chart and trial uses it. Naegele's rule is the mental math: add a year, subtract three months, add seven days. IVF dating is exact because fertilization day is known: a day-5 blastocyst is 19 days into the fictional LMP count (14 + 5). First-trimester CRL is the most accurate biometry (Β±5 days at 7-13 weeks) because early embryos grow uniformly; later measurements reflect genetics and nutrition, so dating shifts to the earliest reliable ultrasound.
Worked Example
LMP January 1, 2026; today April 17: 106 days = 15w 1d, second trimester, EDD October 8 (Jan 1 + 280 days). Conception February 14: LMP-equivalent January 31, EDD November 7. Day-5 transfer March 10: LMP-equivalent February 19, EDD November 26. CRL 40 mm today: GA = 8.052 Γ β40 + 23.73 = 8.052 Γ 6.32 + 23.73 = 74.7 days = 10w 5d. I verified the calculator reproduces these conversions. When LMP and first-trimester ultrasound disagree by more than 5-7 days, guidelines redating to the ultrasound, because the embryo does not lie about its age the way cycle memory can.
Which Dating Wins
| Method | Accuracy | Notes |
|---|---|---|
| IVF transfer/retrieval | Exact | Fertilization day known; never redated |
| CRL ultrasound 7-13 weeks | Β±5 days | Gold standard; redates LMP if off by >5-7 days |
| LMP, regular cycles | Β±1-2 weeks | Good if cycles regular and date certain |
| LMP, irregular cycles | Poor | Ultrasound dating needed |
| Second-trimester biometry | Β±10-14 days | Fallback when no early dating exists |
Redating rules prevent churn: first-trimester ultrasound replaces LMP when the difference exceeds 5 days (under 9 weeks) or 7 days (9-13w6d); second-trimester scans replace it beyond 10-14 day differences. After 22 weeks, redating is discouraged because growth variation swamps dating precision. For everything gestational age guides, our due date calculator expands the EDD math, our fundal height tracker follows physical growth, and our fetal weight percentile calculator interprets the anatomy scan.
Trimesters and What They Gate
First trimester (0-13w6d): organogenesis, highest miscarriage and teratogen risk, nuchal translucency window at 11-13w6d. Second (14-27w6d): anatomy scan at 18-22 weeks, glucose screening at 24-28, viability threshold at 24. Third (28+): growth surveillance, Group B strep at 36-37w6d, weekly testing when indicated from 36. Each screening has a gestational-age window measured in days, which is why "about 20 weeks" is not precise enough and the calculator reports weeks+days.
Common Dating Confusions
"I know exactly when I conceived" still converts to LMP-equivalent by adding 14 days, because every chart speaks LMP. Implantation bleeding mistaken for a period shifts LMP a month late; breastfeeding and recent contraception do the same. IVF pregnancies are never redated by ultrasound: the lab dates beat the scan. And gestational age is not embryonic age: at "8 weeks pregnant" the embryo has existed for about 6 weeks, a distinction that matters for teratogen counseling and not much else.
When Dating Goes Wrong: Real Cases
Dating errors cluster in predictable situations. Irregular cycles are the classic: a 40-day cycle means ovulation near day 26, so LMP dating overstates age by almost two weeks and can trigger a false "post-term" induction if never corrected by ultrasound. Implantation bleeding mistaken for a period shifts everything a month late, producing the alarming "no heartbeat at 8 weeks" that resolves when the dates move. Recent hormonal contraception and breastfeeding both distort the LMP anchor. IVF dating is immune to all of this, which is one reason IVF pregnancies have the most reliable gestational ages in obstetrics. The clinical cost of wrong dates is real: misdated pregnancies get mistimed aneuploidy screening (wrong MoM values), growth restriction diagnosed or missed incorrectly, and inductions scheduled for post-term that is not post-term. When in doubt, the earliest ultrasound is the tiebreaker, and documenting how the EDD was set prevents the confusion from compounding across providers.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
How many weeks pregnant am I?
Count from the first day of your last menstrual period: divide the days since LMP by 7. This calculator does it from LMP, conception, ultrasound CRL, or IVF dates.
Why is pregnancy counted from the last period instead of conception?
Convention: LMP was historically knowable while ovulation day was not, and every chart, trial, and guideline uses it. Add 14 days to a known conception date to get the LMP-equivalent.
How accurate is ultrasound dating?
First-trimester CRL is the gold standard at Β±5 days (7-13 weeks). Accuracy falls later: Β±10-14 days in the second trimester, worse in the third, which is why early dating is preferred.
When should the due date be changed?
When a first-trimester ultrasound differs from LMP by more than 5-7 days, guidelines redating to the scan. Small differences keep the LMP date. After 22 weeks, redating is discouraged.
How is IVF pregnancy dated?
From fertilization: LMP-equivalent is transfer minus 17 days (day-3) or 19 days (day-5), or retrieval minus 14 days. IVF dates are exact and never redated by ultrasound.
What is Naegele's rule?
The mental due-date math: add 7 days to LMP, subtract 3 months, add 1 year. It assumes a 28-day cycle with ovulation on day 14.
What are the trimesters in weeks?
First: 0-13w6d. Second: 14w0d-27w6d. Third: 28w0d to delivery. Key screenings (NT scan, anatomy scan, glucose test) are scheduled by exact gestational age.
How many babies are born on their due date?
Only about 4-5%. About 80% arrive within two weeks either side; 37w0d-41w6d is considered full term. The due date is an estimate, not an appointment.
Can I be redated in the third trimester?
Almost never: growth variation makes late biometry too imprecise for dating. Redating after 22 weeks is discouraged by guidelines.
Does a 35-day cycle change my due date?
Yes, if dating by LMP alone: ovulation likely occurred around day 21, not day 14, so LMP dating overstates age by about a week. A first-trimester ultrasound corrects this.