Health calculator

Due Date Calculator, Pregnancy Timeline & Trimester Tracker

Estimate your pregnancy due date and see week-by-week milestones. Calculate from last menstrual period or conception date with trimester markers.

Published by · Reviewed against primary-source formulas

days

Typical range is 21-35 days

Estimated Due Date

–

Current Week

–

Of pregnancy

Trimester

–

Current phase

Days Remaining

–

Until due date

How CalcMesh estimates a due date

We estimate the due date with Naegele’s rule, which adds 280 days (40 weeks) to the first day of the last menstrual period. According to the American College of Obstetricians and Gynecologists, an ultrasound dating scan is more accurate, especially with irregular cycles.

We let you adjust for cycle length; the assumptions we use are documented in our methodology. This is an estimate, not medical advice.

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.

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.

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This page identifies the inputs, method, and limitations behind its estimates. Calculator outputs are not professional advice and should be checked against the relevant primary source for a consequential decision. This calculator's formula and defaults are drawn from standard published sources for its domain, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.

Inputs, defaults, and authoritative sources
Input Default Source / authority
All inputs Domain-typical defaults Editorial methodology, CalcMesh 2026