Pregnancy Due Date Calculator

How to Calculate Your Pregnancy Due Date

Knowing your estimated due date (EDD) is one of the first things you'll want to figure out after a positive pregnancy test. Your due date is a milestone that helps your healthcare provider track your baby's development, schedule prenatal appointments, and plan for delivery. This calculator uses the two most common methods: the Last Menstrual Period (LMP) method and the Conception Date method.

The Last Menstrual Period (LMP) Method

The LMP method is the standard approach used by most obstetricians worldwide. It is based on Naegele's rule: add 280 days (exactly 40 weeks) to the first day of your last menstrual period. This works because pregnancy is measured from the LMP — even though conception doesn't actually occur until roughly 14 days later at ovulation.

If your cycle length differs from the typical 28 days, a small adjustment is applied: for every day your cycle is longer than 28, one day is added to the due date, and vice versa. For example, with a 35-day cycle, your due date shifts 7 days later than the standard Naegele calculation.

The Conception Date Method

If you know your approximate conception date — perhaps because you were tracking ovulation, used fertility treatments, or conceived via IVF — you can calculate your due date by adding 266 days (38 weeks) to the conception date. This is slightly shorter than the LMP calculation because conception happens about 14 days after the period begins. IVF patients often use embryo transfer date as the reference point, with adjustments based on embryo age (day 3 or day 5).

Understanding Your Pregnancy Timeline

A full-term pregnancy is divided into three trimesters, each with distinct developmental milestones for your baby and physical changes for you:

First Trimester (Weeks 1–12)

The first trimester runs from your LMP through week 12. This is when your baby's major organs and body systems begin forming. Morning sickness, fatigue, and breast tenderness are common. The nuchal translucency ultrasound (around 11–13 weeks) and first-trimester blood screening are typically done during this period to check for chromosomal conditions.

Second Trimester (Weeks 13–27)

Many women describe the second trimester as the most comfortable phase of pregnancy. Morning sickness usually subsides, energy returns, and you'll begin to feel baby's movements (quickening) between weeks 16 and 25. The anatomy scan at week 20 is one of the most anticipated appointments — it checks baby's organs and can reveal the sex if desired.

Third Trimester (Weeks 28–40)

The final stretch of pregnancy runs from week 28 to your due date. Your baby gains most of its weight during this period and moves into a head-down position in preparation for birth. You'll have more frequent prenatal visits, a Group B Strep test around week 36, and monitoring for signs of labor. Most healthcare providers aim for delivery between 39 and 41 weeks.

Key Pregnancy Milestones Explained

Week 12 — End of First Trimester

By week 12 the risk of miscarriage drops significantly and most couples feel comfortable sharing their news. The baby is now about 2.5 inches long and all major organs have formed. Your first-trimester screening should be complete.

Week 20 — Anatomy Scan

The mid-pregnancy ultrasound at 18–22 weeks (often scheduled at exactly 20 weeks) is a detailed examination of your baby's anatomy. The sonographer checks the brain, heart, spine, kidneys, and limbs. Placenta position is also assessed. This is the scan where many parents learn whether they are expecting a boy or a girl.

Week 28 — Third Trimester Begins

Week 28 marks the start of the third trimester and is a meaningful milestone. Babies born at 28 weeks have a very high survival rate with modern neonatal care. Your prenatal visits will become more frequent from here. Your provider will check for gestational diabetes (glucose tolerance test is usually done between 24–28 weeks) and monitor blood pressure for signs of preeclampsia.

Week 37 — Full Term

A pregnancy is considered early term at 37–38 weeks and full term at 39–40 weeks. At 37 weeks your baby's lungs and brain are sufficiently developed for life outside the womb in most cases. While preterm birth before 37 weeks can lead to complications, babies born at 37 weeks generally do very well. Your provider will not typically intervene to delay labor after week 37.

Week 40 — Due Date

Only about 5% of babies arrive exactly on their due date, but it remains the central target of your pregnancy. If labor has not begun by 41 weeks, your provider will usually recommend induction to avoid post-term risks. Around 90% of babies are born within two weeks of the estimated due date.

Factors That Can Affect Your Due Date Accuracy

• Irregular cycles: Women with irregular periods may have less predictable ovulation timing, making LMP-based estimates less accurate.
• Uncertain LMP: If you don't remember the exact date of your last period, an early ultrasound is the best way to date the pregnancy.
• First-trimester ultrasound: A dating ultrasound between 8 and 14 weeks using crown-rump length (CRL) measurements is considered more accurate than LMP dating and can shift your due date by days or even a week or two.
• IVF pregnancies: Dates are calculated from egg retrieval or embryo transfer and are generally more precise than natural conception estimates.
• Multiples: Twin and triplet pregnancies have their own gestational timeline and are typically delivered earlier than singleton pregnancies.

What to Do After Calculating Your Due Date

Once you have your estimated due date, your next steps are:

• Schedule your first prenatal appointment (typically between 8 and 10 weeks).
• Start or continue taking prenatal vitamins with folic acid.
• Review your workplace maternity leave options and begin planning.
• Calculate key deadlines: when to announce, when to schedule your anatomy scan, when to start a birth plan, when to pack your hospital bag.
• Use an organizational app to track appointments, symptoms, and milestones throughout your pregnancy.