🤰Pregnancy Calculator
Calculate your pregnancy due date, gestational age, trimester, and full milestone calendar using LMP (last menstrual period), conception date, ultrasound date, or IVF transfer date. Uses Naegele's rule (+280 days from LMP) as recommended by ACOG.
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Pregnancy Calculator: Due Date, Gestational Age & Milestone Guide
Naegele's rule calculates the estimated due date (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). It assumes a 28-day cycle with ovulation at day 14. Gestational age is measured from LMP, not from conception — so a fetus is technically "2 weeks old" at conception. Only ~5% of babies are born on the exact EDD; 90% arrive between 38 and 42 weeks.
Formula: EDD = LMP + 280 days | Conception → EDD = Conception Date + 266 days | IVF Day-5 → EDD = Transfer + 261 days
| Method | Input | Days to Add |
|---|---|---|
| LMP (Naegele's) | First day of last period | +280 days |
| Conception Date | Ovulation / fertilisation date | +266 days |
| IVF Day-5 Transfer | Blastocyst transfer date | +261 days |
Our pregnancy calculator supports five input methods: LMP (the standard clinical approach), conception/ovulation date, first-trimester ultrasound gestational age (the most accurate method), IVF day-5 blastocyst transfer, and IVF day-3 embryo transfer. All methods convert to an equivalent LMP date internally, then calculate the due date and full milestone calendar from that anchor point. The result includes every major pregnancy milestone from conception through the post-term threshold at 42 weeks.
Understanding Gestational Age: Why It's Counted From LMP
Gestational age is the universally adopted clinical standard for dating a pregnancy. It is counted from the first day of the last menstrual period, not from the date of conception, which is typically 2 weeks later. This means a woman is considered "4 weeks pregnant" at the time her period is missed (when conception occurred approximately 2 weeks earlier). This convention exists because LMP is reliably known while the exact date of ovulation and fertilisation usually is not.
Fertilisation (the union of egg and sperm) typically occurs on the day of ovulation — day 14 of a 28-day cycle — but can occur up to 5 days after intercourse as sperm can survive in the female reproductive tract for up to 5 days. Implantation follows 6–10 days after fertilisation, and the pregnancy hormone hCG becomes detectable in urine approximately 10–14 days post-ovulation (28 days gestational age).
First-Trimester Ultrasound: The Most Accurate Dating Method
While LMP-based dating is the standard starting point, first-trimester ultrasound between 8 and 13 weeks 6 days provides the most accurate gestational age assessment. At this stage, fetal crown-rump length (CRL) measurement has a measurement error of ±5–7 days, which is substantially more accurate than LMP dating (which can be uncertain in women with irregular cycles, recent hormonal contraception, or perimenopause).
ACOG and SMFM recommend that if a first-trimester CRL measurement differs from LMP-based dating by more than 7 days, the ultrasound date should supersede the LMP date. Between 14 and 23 weeks, femur length and biparietal diameter measurements are used; accuracy decreases to ±2–3 weeks in the third trimester, where ultrasound dating is unreliable for establishing gestational age from scratch.
Key Pregnancy Milestones Explained
The first trimester (weeks 1–13) encompasses the most critical period of organogenesis — all major organs and body systems form during weeks 3–10. Neural tube closure occurs by week 6. The fetal heartbeat is typically detectable by transvaginal ultrasound from 6 weeks. The nuchal translucency scan at 11–13 weeks screens for Down syndrome and other chromosomal abnormalities.
The second trimester (weeks 14–27) is often described as the most comfortable. Fetal movements become perceptible (quickening) from approximately 16–20 weeks. The anatomy scan at 18–20 weeks evaluates fetal structures in detail. Fetal viability — the threshold at which neonatal survival outside the womb becomes possible with intensive care — is considered to begin at 22–24 weeks.
The third trimester (weeks 28–40) involves rapid fetal weight gain (approximately 0.5 lb/week in the final month) and preparation for delivery. Full term is defined as 39–40 weeks 6 days; early term is 37–38 weeks 6 days. Delivery before 37 weeks is preterm; after 42 weeks is post-term and warrants consideration of labour induction to reduce risks.
Frequently Asked Questions
How is the pregnancy due date calculated?
The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation at day 14. If your cycle is shorter or longer, the due date may be adjusted. First-trimester ultrasound (8–13 weeks) provides more accurate dating than LMP alone and should override LMP dates when there's a discrepancy of more than 7 days. Only about 5% of babies are born exactly on the due date.
What is gestational age vs fetal age?
Gestational age (GA) is measured from the first day of the last menstrual period — the universal clinical standard. Fetal age (also called embryonic age or developmental age) is counted from actual conception, approximately 2 weeks after LMP. So a fetus with a gestational age of 10 weeks has an actual developmental age of about 8 weeks. When your doctor or ultrasound report says "10 weeks pregnant," they mean 10 weeks gestational age, not 10 weeks since conception. This can cause confusion — you are "pregnant" for 40 gestational weeks, but the embryo/fetus only develops for 38.
How do I calculate my due date after IVF?
For IVF pregnancies, due date calculation depends on the embryo transfer day. For a day-5 blastocyst transfer, add 261 days to the transfer date (equivalent to LMP + 280 days, since a blastocyst is 5 days old at transfer and conception equivalent is 14 days after LMP — so 280 − 14 − 5 = 261). For a day-3 embryo transfer, add 263 days. These dates are highly accurate because the exact developmental stage of the embryo is known. IVF pregnancies are dated by transfer date, not LMP.
What are the three trimesters of pregnancy?
Pregnancy is divided into three trimesters: First Trimester (weeks 1–13) — the embryonic/early fetal period when all major organs form; highest miscarriage risk is in weeks 6–10. Second Trimester (weeks 14–27) — typically the most comfortable; fetal movements begin (16–20w), anatomy scan at 18–20w, fetal viability threshold at 22–24w. Third Trimester (weeks 28–40+) — rapid fetal growth and weight gain; full term reached at 39w; labour induction may be considered at 41–42w.
What if my due date has passed?
Most healthy pregnancies end between 38 and 42 weeks. Going past your due date (40 weeks) is common — approximately 10–15% of pregnancies reach 41 weeks. Post-term pregnancy (42+ weeks) carries increased risks including placental insufficiency, meconium aspiration, and stillbirth. Most obstetric guidelines recommend offering labour induction between 41 and 42 weeks. At 42 weeks, active management is strongly recommended. Consult your healthcare provider about monitoring and timing of delivery if you pass your due date.