Pregnancy Calculator
Calculate your due date from LMP, conception date, or IVF transfer date. See gestational age, trimester, and pregnancy milestones.
This calculator provides estimates only and does not constitute medical advice. Consult your healthcare provider or midwife for personalized pregnancy care. Due dates are estimates — most babies are born between 38 and 42 weeks.
How Pregnancy Due Dates Are Calculated
The most widely used method for calculating a pregnancy due date is Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This rule assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. The 280-day figure is derived from the average length of human pregnancy: approximately 266 days from conception (38 weeks) plus 14 days from the LMP to ovulation.
If your cycle is not 28 days, the estimated due date is adjusted. For a 35-day cycle, ovulation occurs around day 21 rather than day 14, so 7 extra days are added to the due date estimate.
Naegele's Rule: Due Date = LMP + 280 days (adjusted for cycle length)
Calculation Methods Compared
| Method | Input Required | Formula | Best For |
|---|---|---|---|
| Last Menstrual Period (LMP) | First day of last period + cycle length | LMP + 280 days (±cycle adjustment) | Natural conception, regular cycles |
| Conception Date | Known or estimated date of conception | Conception + 266 days | When ovulation date is known |
| IVF — 3-day embryo | Transfer date | Transfer + 263 days | IVF patients with day-3 transfer |
| IVF — 5-day blastocyst | Transfer date | Transfer + 261 days | IVF patients with day-5 transfer |
The Three Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental milestones:
| Trimester | Weeks | Key Events |
|---|---|---|
| First | Weeks 1–13 | Implantation, all major organs form, highest miscarriage risk, morning sickness common |
| Second | Weeks 14–27 | Rapid growth, fetal movement (quickening), anatomy ultrasound at week 20, lower miscarriage risk |
| Third | Weeks 28–40+ | Lung maturation, maximum growth, preparation for birth, frequent prenatal visits |
Key Pregnancy Milestones
Several gestational ages mark clinically significant thresholds:
- Week 6–8: Heartbeat detectable on ultrasound.
- Week 10: All major organs have formed; embryo is now called a fetus.
- Week 12: End of first trimester; risk of miscarriage drops significantly. Many women choose this as the point to announce the pregnancy.
- Week 20: Mid-pregnancy anatomy scan (detailed ultrasound) checks for structural anomalies and can determine sex.
- Week 24: Viability threshold — with intensive neonatal care, some babies born at this gestational age can survive. Survival rates increase significantly with each additional week.
- Week 28: Third trimester begins. Group B streptococcus testing typically done around week 36.
- Week 37: "Full term" begins (37–42 weeks).
- Week 40: Estimated due date (EDD).
- Week 42: Post-term; most providers recommend induction if labor has not begun.
How Accurate Is Your Due Date?
Research shows that only about 4% of pregnancies end exactly on the calculated due date. Approximately 70% of births occur within 10 days of the due date, and 95% within 3 weeks. The term "due date" is somewhat misleading — "due month" would be more accurate.
First-trimester ultrasound is the gold standard for confirming gestational age and is more accurate than LMP-based calculation, particularly for women with irregular cycles. Your healthcare provider may adjust your due date based on ultrasound measurements.
Factors that can influence gestational length include: maternal age (older mothers statistically carry slightly longer), race/ethnicity, number of previous pregnancies, whether the conception was spontaneous or assisted, and genetics (pregnancy length runs in families).
Frequently Asked Questions
How is the due date calculated from the last period?
The standard method is Naegele's rule, named after German obstetrician Franz Karl Naegele (1778–1851). The rule adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator adjusts accordingly — for example, a 35-day cycle shifts the due date 7 days later than a 28-day cycle. Naegele's rule is used by virtually all obstetric guidelines worldwide as the starting point for pregnancy dating.
How accurate is the calculated due date?
Only about 4–5% of babies are born on their exact due date. Most births occur in a window between 38 and 42 weeks — the so-called "full term" range (37–42 weeks). A due date is best understood as the middle of a 4-week window, not a fixed target. Early ultrasound (before 14 weeks) can refine the estimate by directly measuring the embryo or fetus, and is more accurate than LMP dating for women with irregular cycles. Your healthcare provider may adjust your due date based on ultrasound findings.
What is gestational age versus fetal age?
Gestational age is counted from the first day of the last menstrual period — so you are technically "2 weeks pregnant" before conception even occurs. Fetal age (also called conceptional age or embryonic age) starts from the moment of fertilization and is approximately 2 weeks less than gestational age. All standard pregnancy milestones — trimesters, anatomy scans, viability — are defined using gestational age, as are the dates shown in this calculator. When your doctor says "you are 12 weeks," they mean 12 weeks of gestational age.
How does IVF change the due date calculation?
With IVF, the fertilization date is known precisely. For a 3-day embryo transfer (the embryo was fertilized 3 days before transfer), the due date is 263 days after the transfer date (266 days from fertilization). For a 5-day blastocyst transfer, it is 261 days after transfer (266 days from fertilization). Gestational age at transfer is 2 weeks 3 days for a 3-day embryo and 2 weeks 5 days for a 5-day blastocyst, because gestational age is counted from the equivalent LMP — roughly 2 weeks before fertilization.
What are the pregnancy trimesters?
A full-term pregnancy is divided into three trimesters: First trimester (weeks 1–13): fertilization, implantation, and rapid organogenesis. All major organs and structures form during this period. The risk of miscarriage is highest in the first trimester. Second trimester (weeks 14–27): rapid fetal growth, movement felt by the mother (quickening), and the mid-pregnancy anatomy ultrasound at around week 20. Third trimester (weeks 28–40+): continued growth, lung maturation, and preparation for birth. Viability — the point at which a premature baby can survive outside the womb with intensive care — is generally considered to begin around week 24.
Related Calculators
- Ovulation Calculator — Predict fertile window from LMP and cycle length
- Period Calculator — Predict next period date
- Age Calculator — Calculate exact age in years, months, and days