Pregnancy Calculator

How Pregnancy Calculator Works

Due Date = LMP + 280 days (40 weeks) | Gestational Age = Today − LMP | Conception ≈ LMP + 14 days

LMP (Last Menstrual Period)

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.

280 days (40 weeks)

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

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

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 Date

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

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

Common Uses

• •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.

Understanding the Result

Worked Examples

Frequently Asked Questions

How accurate is the due date calculated from LMP?

The due date is a statistical estimate, not a precise prediction. Only about 5% of babies are born on their exact due date. Normal delivery can occur anywhere between 37 and 42 weeks. First-trimester ultrasound (crown-rump length at 8-13 weeks) provides the most accurate dating with an error margin of ±5-7 days. If your ultrasound dating differs significantly from your LMP date, the ultrasound-based date is typically considered more reliable.

What if my menstrual cycle is longer or shorter than 28 days?

If your cycle is consistently longer or shorter, adjust the cycle length input accordingly. The calculator shifts the due date by the difference from 28 days. For example, a 35-day cycle shifts ovulation to day 21, adding 7 days to the due date. For irregular cycles (varying by 7+ days between cycles), early ultrasound dating is recommended for the most accurate due date.

When does each trimester begin and end?

The first trimester spans weeks 1 through 13, starting from LMP. The second trimester covers weeks 14 through 26. The third trimester runs from week 27 through delivery (typically weeks 27-40). Each trimester brings different developmental milestones and prenatal care focuses. Your healthcare provider will recommend specific screenings and tests timed to each trimester.

What are the key pregnancy milestones to track?

Key milestones include: end of first trimester (week 13) when organ formation is complete; viability threshold (week 24) when the fetus has a chance of survival with medical support; full term (week 37) when the baby is considered early term and delivery is no longer considered preterm; and the estimated due date (week 40). The 20-week anatomy scan between weeks 18-22 is also a major prenatal milestone.

Why is pregnancy dated from the last menstrual period and not from conception?

LMP dating has been the clinical standard since the early 1800s because the first day of the last period is usually the most reliably known date. Conception is rarely known with certainty — sperm can survive 5-7 days in the reproductive tract, and ovulation timing varies even in regular cycles. Using LMP provides a consistent reference point across all pregnancies, even if it means the first 2 weeks of "pregnancy" occur before conception actually happens. This convention ensures all healthcare providers, ultrasound machines, and growth charts use the same dating language.

What should I do if my due date changes after an ultrasound?

It is common and normal for the due date to be adjusted after a first-trimester ultrasound (typically done at 8-13 weeks). If the ultrasound-based date differs from the LMP-based date by 7 or more days, healthcare providers typically adopt the ultrasound date as it is considered more accurate (±5-7 days) than LMP dating. Late first-trimester and second-trimester ultrasounds are less accurate for dating, so providers are less likely to change the due date based on later scans. Do not be alarmed if your due date shifts — early ultrasound dating is the gold standard.

What does "full term" actually mean, and why does it matter?

Full term is now defined more precisely than simply "40 weeks." Early term: 37 weeks 0 days through 38 weeks 6 days. Full term: 39 weeks 0 days through 40 weeks 6 days. Late term: 41 weeks 0 days through 41 weeks 6 days. Post-term: 42 weeks and beyond. This distinction matters because babies born at 37-38 weeks (early term) have higher rates of respiratory issues, feeding difficulties, and NICU admissions compared to babies born at 39-40 weeks (full term). For this reason, elective inductions and scheduled C-sections are typically not performed before 39 weeks unless medically indicated.

Pro Tips

• →Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception — this means you are already "2 weeks pregnant" at conception. This is the standard medical convention used worldwide.
• →If you have irregular cycles, your due date may be adjusted after a first-trimester ultrasound, which measures the crown-rump length of the embryo and is more accurate than LMP-based dating (±5-7 days vs ±2-3 weeks).
• →Book your first prenatal appointment as soon as you confirm pregnancy — ideally before 8-10 weeks. Early prenatal care is associated with better outcomes for both mother and baby.
• →Consider downloading a pregnancy tracking app alongside using this calculator; weekly fetal development information helps you understand what is happening at each stage.

Limitations to Know

• •This calculator uses Naegele's rule (LMP + 280 days, adjusted for cycle length) which provides an estimated due date. Only about 4-5% of babies are born on their exact due date.
• •Irregular menstrual cycles, late ovulation, or uncertain LMP dates can significantly affect accuracy. A first-trimester ultrasound is the gold standard for dating a pregnancy.
• •This calculator is for informational purposes only and is not a substitute for professional prenatal care. Always consult your healthcare provider for personalized pregnancy dating and management.
• •Adjustments for IVF pregnancies or pregnancies achieved through assisted reproductive technology are not modeled; those due dates are typically calculated from the embryo transfer date.
• •Trimester boundaries are approximate: first trimester (weeks 1-13), second trimester (weeks 14-26), third trimester (weeks 27-40). Some guidelines use slightly different cutoff points.

Sources

• ACOG - Method for Estimating Due Date
• WHO - Pregnancy and Prenatal Care