Pregnancy Calculator — Due Date, Trimester & Milestones
Track your pregnancy week by week. Calculate due date, current trimester, gestational age, and key milestones from your last menstrual period.
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The Formula
The Estimated Due Date (EDD) is calculated by adding 280 days (40 weeks) to the first day of the Last Menstrual Period, following Naegele's Rule — a dating convention established by German obstetrician Franz Naegele in the early 1800s. Gestational age is then tracked forward from LMP as the standard clinical measure. Conception is estimated at roughly 14 days after LMP, corresponding to ovulation in a typical 28-day cycle.
Variable Definitions
Last Menstrual Period
The first day of your last menstrual period — the standard clinical reference point for dating a pregnancy. Gestational age is counted from LMP, not from conception.
Full Gestation Length
Standard human gestation averages 280 days from LMP. A pregnancy is considered full term between 37 and 42 weeks. Only about 5% of births occur on the exact due date.
Gestational Age
The current duration of pregnancy measured from LMP in weeks and days. At 40 weeks + 0 days the pregnancy reaches full term. Calculated as today minus LMP date.
Trimester Classification
Pregnancy is divided into three trimesters: 1st (weeks 1-13) with organ formation and highest miscarriage risk; 2nd (weeks 14-26) with fetal movement and anatomy screening; 3rd (weeks 27-40) with lung maturation and rapid growth.
Conception (Ovulation) Estimate
Estimated date of conception, approximately 14 days after LMP in a 28-day cycle. Conception occurs when a sperm fertilizes the egg within 12-24 hours of ovulation.
How to Use This Calculator
- 1
Enter the first day of your last menstrual period using the date picker.
- 2
Adjust the cycle length if your average menstrual cycle is shorter or longer than 28 days (range 20-45 days).
- 3
Review your estimated due date, current gestational age, and trimester classification.
- 4
Reference the milestone dates for 1st trimester end, viability, and full term to plan prenatal care appointments.
Quick Reference
| From | To |
|---|---|
| Week 4 | Implantation complete; pregnancy hormone (hCG) detectable |
| Week 8 | Heartbeat visible on ultrasound; basic limb buds forming |
| Week 12 | End of 1st trimester; all major organs formed; miscarriage risk drops significantly |
| Week 16 | Baby can make facial expressions; sex organs distinguishable |
| Week 20 | Anatomy scan; halfway point; fetal movement often felt |
| Week 24 | Viability threshold; lungs begin surfactant production; baby responds to sound |
| Week 28 | 3rd trimester begins; eyes open; brain wave activity resembles newborn |
| Week 32 | Baby practices breathing; most major systems mature |
| Week 37 | Full term — baby is considered early term; delivery not premature |
| Week 40 | Estimated due date; average pregnancy length |
Common Applications
- Track your pregnancy week by week and understand which trimester you are in for prenatal care planning.
- Plan maternity leave, baby showers, and birth preparations around key milestone dates.
- Share your estimated due date and gestational progress with your healthcare provider at prenatal visits.
The 40-week pregnancy timeline from LMP, divided into three trimesters with key milestone markers for prenatal planning.
Understanding the Concept
Pregnancy is conventionally dated from the first day of the Last Menstrual Period (LMP), a system established by German obstetrician Franz Naegele in the early 1800s. Under Naegele's Rule, the Estimated Due Date (EDD) is calculated as LMP plus 280 days, or 40 weeks. This convention persists because the LMP date is usually known, whereas the exact date of conception is rarely certain. Gestational age — the standard clinical measure of pregnancy progression — is expressed in completed weeks and days from LMP. At 40 weeks and 0 days, the pregnancy reaches its estimated due date. The 40-week journey is divided into three trimesters, each marking distinct developmental milestones for both the baby and the mother. The first trimester (weeks 1-13) is a period of rapid organogenesis — the baby's heart begins beating around week 6, and all major organs are formed by week 12. This trimester carries the highest risk of miscarriage, and many women experience fatigue, nausea, and breast tenderness. The second trimester (weeks 14-26) is often called the 'golden period' as energy returns and fetal movement becomes noticeable (quickening, typically felt around weeks 18-22). The 20-week anatomy scan is a major prenatal milestone. After week 24, the fetus reaches viability — the point at which survival outside the womb is possible with intensive medical support. The third trimester (weeks 27-40) focuses on rapid brain and lung development, significant weight gain, and preparation for birth. At 37 weeks, the baby is considered early term; full term spans 39-40 weeks for optimal outcomes. Understanding these milestones helps expectant parents plan prenatal care, maternity leave, and birth preparations with confidence.
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