Pregnancy Due Date Calculator

Naegele's rule: why due dates are counted from LMP

Modern obstetrics counts pregnancy as 280 days (40 weeks) from the first day of your last menstrual period (LMP), even though conception happens about 2 weeks afterLMP. The convention was named for German obstetrician Franz Karl Naegele in the early 1800s. It survives because LMP is a date most patients can remember, while the exact date of conception usually isn't known.

The math: due date = LMP + 280 days, or equivalently LMP + 9 months + 7 days. Adjust for cycle length: if your cycles are 32 days (4 days longer than the assumed 28), ovulation happens later, so the due date shifts later by ~4 days. The calculator handles this automatically when you set the cycle slider.

Three ways to date a pregnancy

1. LMP (Last Menstrual Period)

Most common method. Requires knowing the first day of your last period. Accurate for 28-day regular cycles; less accurate if your cycles are irregular, unusually long, or short. The calculator's cycle-length slider compensates.

2. Conception date

If you tracked ovulation (with ovulation predictor kits, BBT charts, or fertility apps) and know the date conception happened, this is more precise than LMP for irregular cycles. Due date = conception date + 266 days.

3. Ultrasound dating

The most accurate method when available, especially in the first trimester. The ultrasound measures the embryo or fetus (typically crown-rump length for 8–13 weeks; biparietal diameter and femur length later) and compares to standard growth curves. First-trimester dating is accurate to within ±5–7 days.

Later ultrasounds (second/third trimester) are less precise for dating because normal growth variation widens. If your LMP-based date and a first-trimester ultrasound disagree by more than 7 days, the ultrasound usually wins.

The trimesters and what happens in each

First trimester (weeks 1–13)

Major organ development. The embryo becomes a fetus around week 11. Heartbeat detectable by ultrasound around week 6. Symptoms peak: nausea, fatigue, breast tenderness, food aversions. Highest miscarriage risk (~10–15% of confirmed pregnancies, mostly in the first 12 weeks). First prenatal visits, NIPT genetic screening, dating ultrasound typically happen here.

Second trimester (weeks 14–27)

Often called the “honeymoon” trimester — nausea typically resolves, energy returns. Anatomy ultrasound around 18–22 weeks (sex usually visible if wanted). Quickening (first felt fetal movement) typically 16–22 weeks for first pregnancies, earlier for subsequent. Glucose tolerance test for gestational diabetes around 24–28 weeks.

Third trimester (weeks 28–40+)

Rapid growth. Lungs maturing. Group B Strep screening around 35–37 weeks. Prenatal visits become more frequent (every 2 weeks, then weekly toward the end). Braxton Hicks contractions (practice contractions) may start. The baby usually settles into head-down position by 36 weeks.

Term, preterm, post-term — definitions

• Preterm: birth before 37 weeks. Comes with NICU and developmental risks; severity depends on how early.
• Early term: 37 0/7 to 38 6/7 weeks. Generally healthy but slightly higher complications than full term.
• Full term: 39 0/7 to 40 6/7 weeks. The lowest-risk window.
• Late term: 41 0/7 to 41 6/7 weeks. Slightly elevated risks; many providers induce.
• Post-term: 42 weeks or later. Most providers won't allow pregnancy to extend past 42 weeks because placental function declines and risks rise sharply.

Only ~5% of babies are born on their exact estimated due date. About 80% are born within 2 weeks before or after. The calculator's estimated date is a midpoint, not a deadline.

Twins, triplets, and IVF — different math

Twins: typically born around 36–37 weeks (average), and most providers won't let pregnancy extend past 38 weeks. Standard 40-week math doesn't apply.

Triplets and higher-order multiples: typically born around 32–34 weeks. Custom dating is essential.

IVF pregnancies: dating is precise because the date of embryo transfer is known exactly. Add 38 weeks (266 days) for a fresh transfer at 5 days (blastocyst), or adjust for earlier-stage transfers. Your fertility clinic provides the official due date — use that, not LMP-based calculators.

Important prenatal milestones to plan for

• ~8 weeks: first prenatal visit + dating ultrasound (timing varies by provider).
• 10–13 weeks: optional NIPT (non-invasive prenatal testing) for chromosomal conditions.
• 11–14 weeks: nuchal translucency ultrasound (some providers).
• 18–22 weeks: anatomy scan / level 2 ultrasound. Sex typically visible if you want to know.
• 24–28 weeks: gestational diabetes screening (1-hour glucose challenge test).
• 27–28 weeks: Tdap vaccine recommended for the pregnant person; flu shot if in season.
• 35–37 weeks: Group B Strep screening.
• 36–40 weeks: weekly prenatal visits.

Important note: this is an estimator, not medical advice

This calculator provides an educational estimate based on standard formulas. Always consult a licensed obstetrician, midwife, or family practice provider for personalized prenatal care, accurate dating, and medical decisions. If you have any concerns during pregnancy — bleeding, severe pain, decreased fetal movement after 24 weeks, severe nausea, signs of preeclampsia — contact your provider or go to L&D immediately.

For other timeline tools: our Age Calculatorworks for figuring out a child's age once they're born, and the Date Calculator helps with general date math.