28 days
Longer cycles push the due date later
How is the Due Date Calculated?
Also searched as: due date calculator free | pregnancy due date calculator | when is my due date | pregnancy week calculator Optimized for US users with American units and terminology.
The estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. For cycles shorter or longer than 28 days, the due date is adjusted accordingly. An ultrasound in the first trimester is the most accurate method for dating a pregnancy, as it measures the crown-rump length (CRL) of the embryo.
⚠️ This calculator provides educational estimates only. Always consult a qualified gynecologist or obstetrician for medical guidance and accurate dating of your pregnancy.
Use the Due Date Calculator above — enter your values and get instant results. This free online tool calculates how to calculate due date without any download or signup required. Results update in real time as you type.
Use the Due Date Calculator above — enter your values and get instant results. This free online tool calculates due date calculator from last period without any download or signup required. Results update in real time as you type.
Only about 4% of babies are born on their exact due date. Most are born within a 2-week window around the EDD. A full-term pregnancy is considered 39–40 weeks, but 37–42 weeks is the normal range. First-trimester ultrasound (CRL measurement between 8–14 weeks) is accurate to within ±5 days. LMP-based calculation has an error margin of ±2 weeks. Second and third-trimester ultrasounds are less accurate for dating.
First trimester: Weeks 1–12. The embryo develops all major organs. Most miscarriages occur in this period. Morning sickness is common. Second trimester: Weeks 13–26. Often called the 'golden trimester' — nausea typically improves, energy increases, and the pregnancy becomes visible. Baby begins moving (quickening) around week 18–22. Third trimester: Weeks 27–40. Rapid weight gain, baby turns head-down, Braxton Hicks contractions begin. The lungs are the last organs to mature.
Standard pregnancy care schedule: Week 6–8: Confirm pregnancy and first prenatal visit. Week 11–13: NT scan (nuchal translucency) + NIPT/double/triple marker. Week 18–20: Anomaly scan (detailed ultrasound for structural defects). Week 24–28: Glucose tolerance test (GDM screening), complete blood count, Rh factor. Week 28: Repeat CBC, Rh-negative mothers get anti-D injection. Week 32–36: Growth scan. Week 36+: Weekly monitoring. Your obstetrician may recommend additional tests based on your age and risk factors.
Gestational age is counted from the first day of the last menstrual period — so at conception (week 2), the gestational age is already 2 weeks. This is the standard used by doctors. Fetal age (embryonic age) is counted from conception — it is always approximately 2 weeks less than gestational age. When doctors say 'you are 8 weeks pregnant', they mean 8 weeks gestational age (6 weeks since conception). Ultrasound reports use gestational age unless otherwise specified.
Ideally, start folic acid (400–800 mcg/day) at least 1–3 months BEFORE conception, not just after you find out you're pregnant. Neural tube defects (spina bifida, anencephaly) occur in weeks 3–4 of pregnancy — before most women know they are pregnant. Women with prior neural tube defect pregnancies may be prescribed 4,000 mcg. Continue through at least the first 12 weeks of pregnancy. Most prenatal vitamins contain the recommended amount. Available OTC in India as folic acid 5mg tablets (consult your doctor for the right dose).