Pregnancy Due Date Calculator — EDD, Weeks Pregnant & Conception Date

A pregnancy due date calculator uses the first day of your last menstrual period, your conception date, or an early ultrasound measurement to estimate when your baby is due — along with how many weeks pregnant you are right now. This tool uses the standard medical calculation (Naegele's Rule, 280 days from LMP) and shows your trimester, remaining days, and major milestones ahead.

How is a pregnancy due date calculated?

The standard method used by nearly every obstetrician worldwide is called Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1812. It's elegantly simple: add 280 days (40 weeks, or 9 months and 7 days) to the first day of your last menstrual period (LMP), and that gives you the estimated due date (EDD).

The formula assumes a 28-day menstrual cycle with ovulation on day 14. In reality, cycles vary — but Naegele's Rule remains the standard because it's a reliable average, it's easy to calculate from a date most women remember, and it's been clinically validated across billions of pregnancies.

Why doctors count from your LMP, not conception

Here's something counterintuitive: when your doctor says you're "6 weeks pregnant," you've only been actually pregnant for about 4 weeks. The medical convention counts from the first day of your last period — which is before ovulation, before fertilization, and before implantation.

There's a reason for this: most women can remember the start of their last period much more accurately than they can pinpoint the moment of conception. The LMP method gives obstetricians a consistent, measurable starting point even when the actual conception date is unknown.

So a "40-week pregnancy" is really about 38 weeks from conception — the extra two weeks are built into the counting convention.

How accurate is a due date calculator?

Here's a fact that surprises most expectant parents: only about 4-5% of babies are born on their actual due date.

Per the American College of Obstetricians and Gynecologists (ACOG), the gestational age window for delivery is classified as:

• 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 0 days and beyond

Within the overall delivery window:

• About 70% of babies are born within 10 days of their estimated due date
• About 50% are born in the week before or the week of their due date
• First-time mothers often deliver slightly later than their calculated date — the median is about 8 days after the EDD for first pregnancies
• Second and later pregnancies tend to deliver closer to the EDD

Due date calculation methods compared

Different methods for estimating due dates have different levels of accuracy. Here's how the main ones compare:

If an early ultrasound dating differs from the LMP-based due date by more than 7 days in the first trimester, obstetricians typically trust the ultrasound and reassign the EDD based on it.

The three trimesters of pregnancy

Pregnancy is divided into three trimesters, each covering roughly 13 weeks. The baby's development focuses on different things in each phase.

What the baby is doing at every stage

Weeks 1-4: The invisible beginning

Technically, you're not pregnant in week 1 (the LMP counting convention). Fertilization occurs around day 14. By the end of week 4, the embryo has implanted in the uterine wall and the first body-plan structures start forming.

Weeks 5-8: Organogenesis

The embryo's organs begin forming in a tightly choreographed sequence. The neural tube (future brain and spinal cord) closes by week 6. The heart starts beating around day 42 (end of week 6). Arms and legs bud. By week 8, the embryo looks recognizably human.

Weeks 9-13: Fetal stage begins

At week 9, the embryo is officially called a fetus. External genitalia begin to differentiate (though sex is already determined — see our article on what happens at fertilization). Fingernails form. By week 13, the baby is about 3 inches long and can make facial expressions.

Weeks 14-20: Rapid growth

The baby doubles in length. Hair follicles form. The mother typically feels the first movements. By week 20, a detailed anatomy scan can check for structural abnormalities.

Weeks 21-27: Viability threshold

By week 24, the baby is considered potentially viable outside the womb (though with significant medical support). The lungs are developing but not yet mature. Brain surface develops the characteristic folds (gyri and sulci).

Weeks 28-36: Final maturation

Rapid weight gain. Brain mass triples. Surfactant (critical for lung function at birth) is produced in increasing amounts. The baby settles into a head-down position usually by week 36. Most babies now would survive outside the womb with little or no medical support.

Weeks 37-42: Delivery window

At weeks 37-38, the baby is considered "early term." At weeks 39-40, "full term." Weeks 41-41+6 are "late term," and 42+ is "post-term." Labor can start naturally anytime in this window. The baby continues to gain about half a pound per week.

Pregnancy weeks vs. pregnancy months

One common source of confusion: pregnancy is often described in both weeks and months, but they don't line up neatly. Here's a quick conversion:

• Month 1: Weeks 1-4
• Month 2: Weeks 5-8
• Month 3: Weeks 9-13 (end of first trimester)
• Month 4: Weeks 14-17
• Month 5: Weeks 18-21
• Month 6: Weeks 22-26 (end of second trimester)
• Month 7: Weeks 27-30
• Month 8: Weeks 31-35
• Month 9: Weeks 36-40 (end of third trimester)

Because a calendar month averages 4.3 weeks, pregnancy is actually about 9.2 months — not an even 9. This is why "9 months pregnant" isn't the same as 40 weeks.

When should your due date be reassessed?

Doctors may update your estimated due date based on new information, usually from early ultrasounds. Reassessment is typical when:

• Your menstrual cycles are irregular, so LMP is unreliable
• You don't remember your LMP exactly
• An early (first trimester) ultrasound measurement differs from the LMP date by more than 7 days
• You conceived via IVF — the transfer date gives a more precise EDD
• You were on hormonal birth control shortly before conception, which may have affected your cycle

Early ultrasound dating (weeks 7-13) is considered more accurate than LMP in these situations because the embryo's crown-rump length grows at a very predictable rate during this period.

Common misconceptions about due dates

Myth: "If I go past my due date, I'll be induced immediately"

Not necessarily. Most obstetricians monitor pregnancies up to 41-42 weeks before considering induction, as long as the baby is healthy and the mother is doing well. Pregnancies going slightly past 40 weeks are completely normal.

Myth: "Due dates are precise"

Even an early ultrasound only estimates the due date within about 5-7 days. Think of your EDD as the center of a 2-week delivery window, not a scheduled appointment.

Myth: "Calculating from conception is more accurate"

Only if you know the exact day of conception, which most people don't. For couples tracking ovulation, this method can be useful. For everyone else, LMP or early ultrasound is more reliable.

Myth: "40 weeks means my pregnancy will be 40 weeks long"

Because 40 weeks is counted from LMP (which is 2 weeks before conception), the actual gestation from conception is about 38 weeks.