Due Date Calculator — Pregnancy Timeline & Trimester Tracker

Q: How accurate is this due date calculator?

Due date calculators provide an estimate based on average pregnancy length. Only about 5% of babies are born on their exact due date. Most births occur within two weeks before or after the estimated date. Your healthcare provider may adjust your due date based on ultrasound measurements, especially in the first trimester when dating is most accurate.

Q: What is Naegele's rule?

Naegele's rule is the traditional method for estimating a due date. Starting from the first day of the last menstrual period, add one year, subtract three months, and add seven days. This assumes a 28-day cycle with ovulation on day 14. The formula is equivalent to adding 280 days (40 weeks) to the LMP date.

Q: When should I see a doctor after a positive test?

Most healthcare providers recommend scheduling your first prenatal visit between weeks 6 and 8 of pregnancy (counted from your last menstrual period). Contact your provider sooner if you experience severe nausea, bleeding, sharp abdominal pain, or have a history of pregnancy complications. Early prenatal care is important for monitoring the health of both parent and baby.

Q: What if my cycle is irregular?

If your menstrual cycle is irregular, a due date calculated from your last period may be less accurate. In this case, an early ultrasound (between weeks 6-12) provides the most reliable dating. You can also use the conception date method if you know approximately when conception occurred. Let your healthcare provider know about cycle irregularity at your first visit.

By CalcMesh Editorial · Reviewed against primary-source formulas

Pregnancy Timeline

Trimesters Overview

First Trimester (Weeks 1-12): Major organ development begins. Morning sickness is common. The heart starts beating around week 6. By week 12, all major organs and body systems have formed.
Second Trimester (Weeks 13-27): Often called the "golden period" as many early symptoms ease. Movement can be felt around weeks 16-20. Gender can typically be determined by ultrasound around week 20.
Third Trimester (Weeks 28-40): Rapid growth and development. The baby gains most of its birth weight. Lungs mature. The baby typically moves into a head-down position in preparation for birth.

Key Milestones

Week 4: Implantation occurs, pregnancy test may turn positive
Week 6: Heartbeat may be detected on ultrasound
Week 12: End of first trimester, risk of miscarriage drops significantly
Week 20: Anatomy scan, halfway point
Week 24: Viability milestone
Week 37: Considered "early term" (full term is 39-40 weeks)
Week 40: Estimated due date

Cycle Length Adjustment

The standard due date calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter, the calculator adjusts accordingly. For example, a 35-day cycle means ovulation likely occurred around day 21, so the due date shifts forward by about 7 days.

Disclaimer: This calculator provides estimates for informational purposes only. Only your healthcare provider can confirm your due date, typically through ultrasound measurements. Consult your doctor or midwife for personalized prenatal care.

Pregnancy Timing in Real Populations

Naegele's Rule (LMP + 280 days) remains the clinical standard but only 5% of births actually occur on the calculated due date, per ACOG's 2017 Committee Opinion. First ultrasound dating (7-13 weeks) is ±5 days accurate, first-trimester ±3-5 days, and second-trimester ±10-14 days. After 22 weeks, LMP becomes more reliable than ultrasound for most patients.

CDC 2022 natality data (3,661,220 U.S. births) shows 60.2% of births occurred at 39-40 weeks, 23.3% at 37-38 weeks (early term), 9.4% preterm (<37 weeks), and 7.1% at 41+ weeks (late/post term). The mean gestational age at birth was 38.6 weeks, and median was 39 weeks. Preterm-birth rates rose to 10.4% in 2022, the highest since 2007 — disparities persist with 14.6% for non-Hispanic Black mothers vs. 9.4% for non-Hispanic white.

Conception timing varies with cycle length. Standard 280-day rule assumes ovulation on day 14 of a 28-day cycle, but studies show only 30% of women ovulate precisely on day 14 — the fertile window spans days 8-20 for most. Accurate dating matters: inaccurate gestational age is a leading cause of iatrogenic preterm delivery (induced labor before 39 weeks), which the March of Dimes estimates adds $1.5 billion annually in avoidable NICU costs.

Sources: ACOG Committee Opinion 700, CDC NCHS natality data, March of Dimes

Methodology & Assumptions

This calculator implements standard formulas drawn from primary-source authorities. Values are point-in-time estimates; consult a licensed professional for high-stakes decisions. See the per-input definitions and source citations below.

How this works

Computations are deterministic and run client-side — no inputs leave your browser. Formulas are derived from standard published formulas for the calculator's domain (mortgage, taxes, energy, conversions, etc.). When the underlying agency publishes updated rates or thresholds we refresh defaults and update the page's lastmod timestamp.

Frequently Asked Questions

How accurate is this due date calculator?

Due date calculators provide an estimate based on average pregnancy length. Only about 5% of babies are born on their exact due date. Most births occur within two weeks before or after the estimated date. Your healthcare provider may adjust your due date based on ultrasound measurements, especially in the first trimester when dating is most accurate.

What is Naegele's rule?

Naegele's rule is the traditional method for estimating a due date. Starting from the first day of the last menstrual period, add one year, subtract three months, and add seven days. This assumes a 28-day cycle with ovulation on day 14. The formula is equivalent to adding 280 days (40 weeks) to the LMP date.

When should I see a doctor after a positive test?

Most healthcare providers recommend scheduling your first prenatal visit between weeks 6 and 8 of pregnancy (counted from your last menstrual period). Contact your provider sooner if you experience severe nausea, bleeding, sharp abdominal pain, or have a history of pregnancy complications. Early prenatal care is important for monitoring the health of both parent and baby.

What if my cycle is irregular?

If your menstrual cycle is irregular, a due date calculated from your last period may be less accurate. In this case, an early ultrasound (between weeks 6-12) provides the most reliable dating. You can also use the conception date method if you know approximately when conception occurred. Let your healthcare provider know about cycle irregularity at your first visit.