Pregnancy Due Date Calculator

How This Pregnancy Due Date Calculator Works

This calculator estimates your Estimated Due Date (EDD) — the date when your baby is expected to arrive — using one of three methods: Last Menstrual Period (LMP), Conception Date, or IVF Transfer Date. All calculations run entirely in your browser with no data stored or transmitted.

The EDD is the most important date in prenatal care. It helps your doctor schedule routine scans, screenings, and check-ups at the right gestational ages. While only about 5% of babies arrive on their exact due date, knowing your EDD gives a clear framework for the entire pregnancy.

Along with the EDD, the calculator shows your current gestational age in weeks and days (measured from your LMP equivalent), your current trimester, and a table of key milestone dates — the end of each trimester and the due date itself.

How to Use This Calculator

1. Select your method: Choose LMP Date if you know the first day of your last period. Choose Conception Date if you know the approximate date of conception (e.g. from tracking ovulation). Choose IVF Transfer if you had in vitro fertilisation.
2. Enter the date: Type or pick the relevant date in the date field. The field label changes to match your chosen method.
3. Set cycle length (LMP only): If using LMP, enter your average menstrual cycle length in days. The default is 28. Valid range is 21 to 35 days.
4. Select embryo type (IVF only): Choose Day-3 (cleavage stage) or Day-5 (blastocyst stage) to get the correct due date.
5. Click Calculate: Your EDD, gestational age, trimester, and milestone dates appear immediately.

Formulas and Logic Explained

LMP Method — Naegele's Rule

The most widely used method is Naegele's Rule, developed by Franz Naegele in 1812 and still used by obstetricians worldwide:

280 days equals exactly 40 weeks, or roughly 9 months and 7 days. If your cycle is not 28 days, this calculator adjusts the EDD by the difference. For a 30-day cycle, 2 days are added (ovulation occurs later); for a 25-day cycle, 3 days are subtracted.

Adjusted formula: EDD = LMP + 280 + (cycle length − 28) days

Conception Date Method

If you know your conception date (from ovulation tracking or intercourse timing):

266 days equals 38 weeks — the duration from fertilisation to delivery. The equivalent LMP is derived as conception date minus 14 days (assuming ovulation at mid-cycle).

IVF Transfer Date Method

For IVF pregnancies, the embryo's age at the time of transfer is already known, so the formula adjusts accordingly:

A Day-3 embryo is already 3 days old at transfer, so 3 days are subtracted from the 266-day post-conception term (266 − 3 = 263). A Day-5 blastocyst gives 266 − 5 = 261 days.

Gestational Age Calculation

Gestational age is counted from the equivalent LMP date (not from conception). The calculator computes the number of full days between your equivalent LMP and today, then divides by 7 to get weeks and days:

Pregnancy Trimesters Explained

Pregnancy is divided into three trimesters, each covering a different phase of fetal development:

• 1st Trimester (weeks 1–13): The embryo implants and all major organ systems begin forming. The risk of miscarriage is highest in this period. Most women experience morning sickness, fatigue, and breast tenderness. The 12-week dating scan typically confirms the EDD.
• 2nd Trimester (weeks 14–27): Often called the "honeymoon trimester" — morning sickness usually eases and energy returns. The fetus grows rapidly; movements (quickening) are felt around weeks 18–20. The anatomy scan at 20 weeks checks for structural abnormalities.
• 3rd Trimester (weeks 28–40): The fetus gains weight, develops lungs, and prepares for birth. Braxton Hicks contractions may begin. A baby born from 37 weeks is considered full-term. Most routine antenatal visits happen weekly after week 36.

Milestone dates shown in the calculator correspond to: end of week 13 (end of T1), end of week 27 (end of T2), and the EDD at week 40 (end of T3).

Worked Examples

How to Interpret Your Results

Your Estimated Due Date is the date at 40 complete gestational weeks. It is the midpoint of a normal delivery window — births between 39 weeks 0 days and 40 weeks 6 days are considered full-term. Early-term births (37–38 weeks 6 days) are also normal. Only about 5% of babies arrive on the exact EDD.

Your gestational age is used by your doctor to schedule routine screenings: the 10–13 week combined test (nuchal translucency + blood tests), the 18–20 week anatomy scan, the 24–28 week glucose tolerance test, and the 36-week Group B Strep screen.

Important: If your first-trimester ultrasound shows a different gestational age than this calculator suggests, your doctor will likely re-date the pregnancy based on the ultrasound (crown-rump length). Ultrasound dating between 8 and 13 weeks is considered more accurate than LMP-based dating.

Calculator Category

This tool belongs to Health Calculators. Browse similar tools for related calculations.