Pregnancy Due Date Calculator

Calculate your expected delivery date, current trimester, and week-by-week pregnancy timeline from your last menstrual period

Calculate by
days
Your Results

Enter your last menstrual period date and click Calculate.

Pregnancy Timeline
TrimesterWeeksKey Milestones
1st TrimesterWeeks 1–12Implantation, heartbeat detected (~week 6), major organs form
2nd TrimesterWeeks 13–26Baby moves (~week 18–20), gender visible on ultrasound, rapid growth
3rd TrimesterWeeks 27–40Baby gains weight, lungs mature, head engages, birth preparation
Frequently Asked Questions
The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle. If your cycle is longer or shorter, the calculator adjusts by adding or subtracting the difference from 28 days.
Only about 5% of babies are born exactly on their due date. Most births occur within 2 weeks before or after the estimated due date. A full-term pregnancy is considered anywhere from 39 to 40 weeks and 6 days. An early ultrasound (between 8–13 weeks) provides the most accurate dating.
LMP stands for Last Menstrual Period. Since the exact date of conception is rarely known, doctors count from the first day of your last period. Ovulation typically occurs about 14 days after LMP, so actual conception is roughly 2 weeks after the LMP date.
The first trimester runs from weeks 1–12, the second from weeks 13–26, and the third from weeks 27–40. The first trimester is the most critical for fetal development, when major organs and systems are forming.
If you don't know your LMP date, you can use the conception date option in this calculator if you know when conception occurred. Alternatively, a doctor can estimate gestational age through an ultrasound, which measures the size of the embryo or fetus.
Naegele's Rule is the standard medical formula for estimating a pregnancy due date. Formulated by German obstetrician Franz Karl Naegele in 1812, it calculates the estimated due date (EDD) as: first day of LMP + 280 days (40 weeks). The formula assumes a 28-day cycle and that ovulation occurs on day 14. Despite its age, Naegele's Rule remains the primary dating method used by clinicians worldwide.
Pregnancy is counted from the first day of the last menstrual period (LMP) rather than from conception because the exact date of conception is rarely known with certainty. The LMP date, by contrast, is something most people can recall or record. In a standard 28-day cycle, ovulation occurs around day 14, so actual conception typically happens approximately 2 weeks after LMP.
Due date predictions are an estimate, not a guarantee. Only about 5% of babies are born on their exact due date. Most babies are born within 2 weeks before or after the estimated due date — the normal range for a full-term birth is 37 to 42 weeks of gestation. A first-trimester ultrasound (before 14 weeks) has a margin of approximately ±5–7 days.
Earlier ultrasounds are more accurate for establishing gestational age. In the first trimester (6–13 weeks), crown-rump length (CRL) measurement provides a dating accuracy of approximately ±5–7 days. By the second trimester, accuracy reduces to approximately ±10–14 days. In the third trimester, accuracy is only ±2–3 weeks.
A pregnancy that continues beyond 42 weeks of gestation is classified as post-term. Post-term pregnancies carry increased risks including placental insufficiency, meconium aspiration, and increased risk of stillbirth. Most obstetric guidelines recommend consideration of labour induction between 41 and 42 weeks. Management decisions should be made in consultation with your healthcare provider.

About This Pregnancy Due Date Calculator

This free Pregnancy Due Date Calculator uses Naegele's Rule — the same method used by doctors worldwide. Enter the first day of your last menstrual period to get your estimated due date, current weeks pregnant, trimester, and key milestone dates. Supports variable cycle lengths between 21 and 35 days.

Weeks Pregnant Reference

WeeksTrimesterApprox. Size
4 weeks1stPoppy seed
8 weeks1stRaspberry
12 weeks1stLime
16 weeks2ndAvocado
20 weeks2ndBanana
24 weeks2ndCorn
32 weeks3rdSquash
40 weeks3rdWatermelon

Standards & References

Related Health Tools
Age Calculator Period Calculator BMI Calculator Water Intake

Related Articles

In-depth guides and technical articles.

View all →
Health Sep 9, 2026
Your Due Date Is a Distribution, Not a Deadline
Only about 4% of babies arrive on their due date. Here's what the birth timing distribution actually looks like, why early term and full term differ, and how to plan.
Health Jul 6, 2026
Why Your 40-Week Due Date Is Actually 38 Weeks of Pregnancy — Ultrasound Dating, Naegele's Rule, and When Induction Is Recommended
A 40-week pregnancy is actually ~38 weeks of fetal development — gestational age is counted from 2 weeks before fertilisation. First-trimester ultrasound (±5 days accuracy) is more accurate than LMP dating for anyone with irregular cycles. Here's why the due date is a median with an asymmetric range, what the ARRIVE trial says about 39-week induction, and why gestational age accuracy matters most in preterm delivery decisions.
Health Jun 19, 2026
Why Pregnancy Is Counted From Before It Began — The LMP Convention, Gestational Age, and Why Your App Says You're Further Along Than You Think
Gestational age is counted from the last menstrual period — not conception — which means a "6-week pregnancy" is about 4 weeks post-conception, and "40 weeks" includes approximately 2 weeks before fertilization. Here's why different calculators give different due dates (cycle length adjustments, ultrasound overrides), what "39+4 weeks" notation means and why the days portion matters clinically, and why Naegele's 280-day rule may slightly underestimate the average first pregnancy duration.
Health Jun 14, 2026
Planning Leave Around Your Due Date: Why "Count Back From the Due Date" Is Just the Starting Point
"My due date is March 15th" doesn't directly answer "when should I start my leave" — leave planning means working backward from an estimated date, applying jurisdiction-specific policies, and building in a buffer for the very real possibility of early arrival. Here's why most leave frameworks let leave begin automatically upon birth regardless of the planned date, why work-handover targets should sit several weeks before the planned leave-start date, and why partner/co-parent leave planning is inherently date-range-based rather than fixed.
Health Jun 12, 2026
Twin Pregnancy Due Dates, Chorionicity, and IVF Dating: How Multiple Pregnancies Differ
Twin pregnancies average 36-37 weeks rather than 40 — meaning a standard due date calculation represents a date most twin pregnancies won't reach. Here's why multiples arrive earlier, how chorionicity affects monitoring and timing, why IVF dating from embryo transfer is the most precise method available, and what vanishing twin syndrome means for dating.