Pregnancy Due Date Calculator 2025 – Exact Due Date & Trimester

How Is Your Pregnancy Due Date Calculated?

Your pregnancy due date — also called the Estimated Due Date (EDD) or Expected Date of Delivery (EDD) — is typically calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP). This method, known as Naegele's Rule, was developed by German obstetrician Franz Karl Naegele in the early 19th century and remains the standard calculation used by healthcare providers worldwide.

It may seem strange to count pregnancy from before conception actually occurred, but it's done this way because the exact date of conception is rarely known, while the start of the last menstrual period is usually well-remembered. In a standard 28-day cycle, ovulation and conception typically occur around day 14 — so 280 days from LMP equals approximately 266 days (38 weeks) from actual conception.

Three Ways to Calculate Your Due Date

Our calculator supports all three clinically accepted methods:

Last Menstrual Period (LMP): The most common method. Add 280 days to the first day of your last period. If your cycle is not 28 days, we adjust accordingly — longer cycles push the due date later, shorter cycles move it earlier.
Conception Date: If you know when conception occurred (through ovulation tracking or IVF), add 266 days (38 weeks) from that date for a more precise estimate.
Ultrasound Dating: An early ultrasound (ideally at 8–12 weeks) measures the baby's crown-rump length (CRL) and is the most accurate method, especially if your cycle is irregular. If the ultrasound date differs from the LMP date by more than 7 days (first trimester) or 14 days (second trimester), doctors typically adjust the due date to match the ultrasound.

The Three Trimesters of Pregnancy

Pregnancy is divided into three trimesters, each with distinct developmental milestones and maternal experiences:

First Trimester (Weeks 1–13): The most critical period for fetal development. All major organs and systems begin forming. Morning sickness, fatigue, and breast tenderness are common. By week 12, your baby is about 2 inches long and has a beating heart. This is when most miscarriages occur, which is why many parents wait until after the first trimester to announce their pregnancy.
Second Trimester (Weeks 14–27): Often called the "golden trimester" — many of the early pregnancy symptoms ease. You'll begin to show visibly and feel the baby's first movements (quickening) around weeks 18–22. This is when the anatomy scan (morphology scan) is performed, usually around week 20. The baby's sex can typically be determined during this scan.
Third Trimester (Weeks 28–40): The final stretch. Your baby gains most of its weight and the organs mature in preparation for birth. You may experience Braxton Hicks contractions, back pain, and increased urination. The baby is considered full-term at 39–40 weeks, though babies born at 37–38 weeks (early term) are also generally healthy.

Important Pregnancy Milestones by Week

Pregnancy is a week-by-week journey with remarkable developments happening constantly. Key milestones include: heartbeat detectable by ultrasound at 6 weeks, risk of miscarriage drops significantly after 12 weeks, gender visible on ultrasound around 16–20 weeks, anatomy scan at 20 weeks, viability milestone at 24 weeks, baby's lungs maturing at 28–32 weeks, and full-term status at 39–40 weeks.

When Should You Call Your Doctor?

Always contact your healthcare provider immediately if you experience: heavy vaginal bleeding, severe abdominal pain or cramping, sudden severe headache, vision changes or seeing spots, sudden swelling of face, hands, or feet, decreased fetal movement after 24 weeks, or any fever over 38°C (100.4°F). These symptoms may indicate complications that require prompt medical attention.

This calculator is for informational purposes only. Always confirm your due date and pregnancy dating with a qualified healthcare provider. Your doctor may adjust your due date based on ultrasound measurements, particularly in the first trimester.

Frequently Asked Questions

How accurate is a due date calculator?

Due date calculators using LMP are accurate to within 1–2 weeks for women with regular 28-day cycles. For irregular cycles, accuracy decreases. First-trimester ultrasound (8–12 weeks) is the most accurate dating method, accurate to within 5–7 days. Only about 5% of babies are born on their exact due date — most arrive within 2 weeks on either side.

What if my cycle is not 28 days?

Our calculator adjusts for cycle length. For each day your cycle differs from 28 days, the due date adjusts accordingly. A 35-day cycle means you ovulate later, so the due date is pushed about 7 days later. A 21-day cycle means earlier ovulation, and the due date moves 7 days earlier. Select your cycle length in the calculator for a more accurate result.

Can my due date change during pregnancy?

Yes. If a first-trimester ultrasound reveals a significant difference from the LMP-based estimate (more than 7 days), your doctor will usually adjust your due date to match the ultrasound. After the first trimester, due dates are rarely changed because ultrasound measurements become less accurate for dating purposes.

What does "full term" mean?

Full term is defined as 39 weeks 0 days through 40 weeks 6 days. "Early term" is 37–38 weeks. "Late term" is 41 weeks. "Post term" is 42 weeks and beyond. Babies born at 37+ weeks are considered viable outside the womb, though outcomes are generally best for babies born at 39–40 weeks when all organ systems are fully mature.

When will I hear the baby's heartbeat?

A baby's heartbeat can typically be detected by transvaginal ultrasound as early as 6 weeks of pregnancy. With an external (abdominal) Doppler device, the heartbeat is usually audible from 10–12 weeks. Hearing the heartbeat for the first time is one of the most emotional moments of pregnancy for most parents.

When will I feel the baby move (quickening)?

First-time mothers (primigravidas) typically feel first movements between 18–22 weeks. Women who have been pregnant before often feel movement earlier, around 16–18 weeks. What you feel at first is often described as butterflies, fluttering, or bubbles. By 24–28 weeks, movements are stronger and more distinct.

What is the anatomy scan and when is it done?

The anatomy scan (also called the 20-week scan or morphology scan) is a detailed ultrasound performed around 18–22 weeks. It checks the baby's organs, limbs, facial features, placenta position, and amniotic fluid levels. It can usually determine the baby's sex (if parents wish to know). This is one of the most important prenatal appointments in the second trimester.

How is IVF due date calculated differently?

For IVF pregnancies, the due date is calculated from the egg retrieval date. For a Day 3 transfer, add 263 days. For a Day 5 (blastocyst) transfer — the most common — add 261 days. IVF due dates are generally more precise than LMP-based calculations because the exact fertilization date is known.

What is Naegele's Rule?

Naegele's Rule is the standard method for calculating a due date: take the first day of the last menstrual period, add one year, subtract three months, and add seven days. This produces the same result as adding 280 days (40 weeks). For example: LMP = January 1 → subtract 3 months = October 1 → add 7 days = October 8 = estimated due date.

Is this calculator medically accurate?

This calculator uses Naegele's Rule and standard gestational age calculations — the same methods used by OB-GYNs worldwide. However, it is intended for informational purposes only and should not replace professional medical care. Always confirm your due date with your healthcare provider, who may use ultrasound measurements for more precise dating.