How accurate is the due date calculation?

The estimated due date (EDD) calculated from LMP is an estimate — only about 5% of babies are born on their exact due date. Most births occur within 2 weeks before or after the EDD. An early ultrasound (7–10 weeks) measuring crown-rump length provides the most accurate dating and may revise the LMP-based estimate if they differ by more than 5–7 days. Dating ultrasound measurements become less accurate as pregnancy progresses.

Why does pregnancy count from LMP and not conception?

Ovulation and conception dates are difficult to know precisely, while the LMP date is easily reported and verified. The convention of counting from LMP dates back to before modern understanding of the menstrual cycle. The LMP-based system adds approximately 2 weeks to the 'true' developmental age of the embryo. When doctors say '8 weeks pregnant,' the embryo is developmentally about 6 weeks old.

Full term is defined as 39 weeks 0 days to 40 weeks 6 days. Early term: 37–38 weeks 6 days. Late term: 41 weeks. Post-term: 42 weeks or later. Babies born before 37 weeks are premature. The ACOG distinguishes these categories because outcomes differ: babies born at 37–38 weeks have higher risk of complications than those born at 39–40 weeks, even though both are technically 'term.'

What is the anatomy scan and when does it happen?

The anatomy scan (mid-pregnancy ultrasound) is typically performed between 18–22 weeks, most commonly at 20 weeks. It checks fetal anatomy (brain, heart, spine, limbs, organs), amniotic fluid levels, placental position, and confirms gestational age. Sex determination is possible at this scan if the baby is in a cooperative position. It's one of the most important routine prenatal assessments.

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When Will Your Baby Arrive? Pregnancy Week Calculator

When will your baby arrive?

What This Does

Pregnancy is counted in weeks from the first day of your last menstrual period (LMP) — not from conception. This standard obstetric convention means your pregnancy officially "starts" about two weeks before conception actually occurs, which can be confusing. The standard gestational period is 40 weeks (280 days) from LMP, though full term is considered 37–42 weeks. Understanding your current week of pregnancy helps you track fetal development, know which prenatal appointments and tests are due, and anticipate when key milestones occur. The first trimester (weeks 1–13) is when the embryo forms all major organs and the risk of miscarriage is highest — most women don't announce until trimester 1 ends. The second trimester (weeks 14–27) is typically the most comfortable period, when the anatomy scan and glucose screening occur. The third trimester (weeks 28–40+) focuses on growth, lung development, and preparation for birth. This calculator determines your estimated due date, current gestational age (weeks and days), trimester, and a complete milestone calendar from LMP through delivery. It supports both LMP-based and conception-date-based calculations, and accounts for irregular cycle lengths using Naegele's rule.

When Should You Use This?

→Finding out your due date from your last menstrual period
→Knowing exactly how many weeks pregnant you are today
→Tracking which trimester you're in and when trimester transitions occur
→Seeing when key prenatal appointments and tests are typically scheduled
→Understanding the full 40-week pregnancy timeline and fetal development milestones

Example Scenario

Sarah's LMP was October 15th. Using Naegele's rule: October 15 + 280 days = July 21 as her estimated due date. Today (December 1) she is 7 weeks and 2 days pregnant, in the first trimester. Key upcoming dates: week 8 (heartbeat clearly visible on ultrasound), week 10 (NIPT/genetic testing option), week 12–14 (nuchal translucency scan), week 20 (anatomy scan).

Pregnancy Calculator

Due Date · Week-by-Week Development · Prenatal Checklist

Results update in real time as you adjust any input.

First Day of Last Period (LMP)

Pre-Pregnancy Weight (lbs)

lbs

Height

ftin

About This Calculator

This pregnancy calculator estimates your due date using Naegele's rule (LMP + 280 days), calculates your current gestational age in weeks and days, and provides a complete pregnancy companion including trimester progress, fetal development milestones, prenatal checklist, and personalised nutrition guidance. All results update in real time as you adjust the date, weight, or height inputs.

The Overview tab includes a stacked trimester bar chart showing completed weeks in each of the three trimesters, with your current trimester highlighted in accent colour and remaining weeks in dark grey. The Development tab plots fetal weight across 13 gestational milestones (weeks 4–40) as an area chart with a reference line at your current week, plus 13 detailed development cards from poppy seed (week 4) to pumpkin (week 40). The Checklist tab displays all 15 prenatal appointments and screenings as a timeline bar chart with green (complete), accent (upcoming), and grey (future) bars.

The Nutrition tab provides personalised extra calorie recommendations by trimester (+0/+340/+450 kcal) and recommended weight gain range based on pre-pregnancy BMI (IOM 2009 guidelines), plus a complete nutrient reference table (folic acid, iron, calcium, DHA, protein, iodine, vitamin D, choline) with daily targets and food sources, and a foods-to-avoid list. Dynamic accent colours shift by trimester: rose (T1), purple (T2), blue (T3), emerald (full term 37–40 weeks). Export to PDF generates a complete report with the prenatal checklist and key insights.

Results are estimates only and do not constitute medical advice. Consult a qualified healthcare professional before making health decisions.