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Pregnancy Due Date Calculator

Estimate your due date, current gestational age, trimester, and conception date using Naegele's rule and ACOG guidelines.

Pregnancy due date inputs

This calculator is for informational purposes only and is not a substitute for professional medical advice. Consult a qualified healthcare provider before making changes to your diet, exercise, or health regimen.

What is a due date and why does it matter?

The estimated due date (EDD), sometimes called the estimated date of delivery or estimated date of confinement, is a single target day 40 weeks after the first day of your last menstrual period (LMP). It is the anchor point for all of obstetric care: week-by-week screening tests, the timing of ultrasound scans, decisions about when to induce a post-term pregnancy, and the definition of "full term" versus preterm all revolve around it.

However, the EDD is very much an estimate. Only about 5% of babies arrive on their calculated due date, and a full-term delivery can happen anywhere from 37 to 42 weeks. The American College of Obstetricians and Gynecologists (ACOG) notes that an ultrasound measurement in the first trimester is the most accurate method of dating; LMP-based estimates like this calculator should be seen as a first approximation that will be refined by prenatal scans.

How this calculator works

We use Naegele's rule, which has been the standard clinical method for estimating delivery dates since the 19th century:

EDD = LMP + 280 days, adjusted by (cycle length − 28) days.

The adjustment matters because Naegele's rule implicitly assumes a 28-day cycle with ovulation on day 14. If your cycles are longer, ovulation — and therefore conception — happens later, so the due date moves back. If your cycles are shorter, it moves forward. The luteal phase (from ovulation to the next period) is relatively constant across individuals at around 14 days, so all the variability in cycle length sits in the pre-ovulation phase.

Gestational age is measured in completed weeks plus days since the LMP, which is why obstetricians say things like "14 weeks and 3 days" or write it as "14+3". We then classify your current status into one of three trimesters using the ACOG boundaries:

  • First trimester: 0 weeks 0 days through 13 weeks 6 days.
  • Second trimester: 14 weeks 0 days through 27 weeks 6 days.
  • Third trimester: 28 weeks 0 days onwards.

Worked example

Imagine your last period started on 1 October 2024 and your average cycle is 30 days.

  • Adjustment = 30 − 28 = +2 days.
  • EDD = 1 October 2024 + 280 days + 2 days = 10 July 2025.
  • Conception date ≈ 1 October 2024 + 14 days + 2 days = 17 October 2024.
  • As of 1 December 2024 (61 days since LMP) you would be 8 weeks 5 days pregnant, firmly in the first trimester.

What happens during each trimester

The first trimester is when most organ formation happens and when the risk of miscarriage is highest. Early symptoms like nausea, fatigue, and breast tenderness are common. The first antenatal appointment and the dating ultrasound typically occur during this period.

The second trimester is often called the "honeymoon" phase. Early pregnancy symptoms ease, fetal movements become noticeable around week 18–22, and the anomaly scan (around 20 weeks) checks for structural issues. Weight gain and baby bump become visible.

The third trimester is the home stretch. The fetus gains most of its weight, lungs mature, and the body prepares for labour. Extra prenatal visits are scheduled, and conditions like gestational diabetes, preeclampsia, and breech position are monitored closely.

Common mistakes

  • Confusing LMP with conception date. LMP is the first day of your last period, about two weeks before conception. Using the conception date by mistake will give an EDD that is two weeks too early.
  • Treating the EDD as a deadline. Only ~5% of babies are born on the EDD. Anything between 37 and 42 weeks is considered full term.
  • Ignoring cycle length. If your cycles are consistently 32 days, a 28-day assumption is off by almost half a week.
  • Skipping the dating ultrasound. Early ultrasound is more accurate than LMP-based dating and should be used to confirm or adjust the EDD.
  • Using this tool instead of a doctor. Online calculators cannot replace real prenatal care.

When to consult a healthcare provider

Always. As soon as you suspect or confirm pregnancy, book an appointment with a qualified healthcare professional — an obstetrician, family doctor, or midwife. Prenatal care reduces the risk of complications for both parent and baby and is the single most important thing you can do during pregnancy. Seek urgent medical attention for severe abdominal pain, heavy bleeding, severe headache with vision changes, reduced fetal movement in later pregnancy, or any symptom that feels seriously wrong. This calculator provides estimates for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Frequently Asked Questions

What is Naegele's rule?
Naegele's rule is a 19th-century formula still used today for estimating the due date: take the first day of the last menstrual period (LMP), add one year, subtract three months, and add seven days. Mathematically this is the same as LMP + 280 days. The rule assumes a 28-day cycle with ovulation on day 14, so the calculator also adjusts for cycle length.
Why is it "40 weeks" when pregnancy is really 38 weeks from conception?
Because obstetric dating counts from the first day of the last menstrual period (LMP), not from conception. Ovulation and conception happen roughly two weeks after the LMP, so "40 weeks pregnant" means 38 weeks since conception. This convention exists because most women know their LMP date but do not know the exact day of ovulation, so LMP is a more reliable anchor.
How accurate is the estimated due date?
According to ACOG, fewer than 5% of babies are born on the exact estimated due date. About 80% of full-term deliveries happen within two weeks either side of the EDD. Ultrasound dating performed between 8 and 13 weeks of gestation is considered the most accurate method and may adjust the EDD by a few days up or down.
What if my cycle is not 28 days?
Cycle length affects when ovulation (and therefore conception) happened. For a 32-day cycle, ovulation is around day 18 rather than day 14, so we add 4 days to the EDD. For a 24-day cycle we subtract 4. The luteal phase (from ovulation to next period) is roughly constant at 14 days, so variations in cycle length are applied to the pre-ovulation phase.
What are the three trimesters?
Using ACOG definitions measured from LMP: first trimester is weeks 0–13+6 (through 13 weeks and 6 days), second trimester is weeks 14+0 through 27+6, and third trimester is 28+0 and beyond. A full-term pregnancy lasts 40 weeks; anything before 37 weeks is considered preterm and after 42 is post-term.
Why does the calculator show weeks and days?
Because obstetricians talk in weeks and days — "14+3" means 14 weeks and 3 days of gestation. This is more precise than whole weeks or calendar months and is the standard notation on medical charts and ultrasound reports.
Is this calculator accurate for IVF pregnancies?
Not directly. For IVF pregnancies the conception date is known precisely (the day of embryo transfer plus a few days for the embryo's age), so clinics calculate the EDD differently — typically transfer date + 266 days, adjusted by embryo age. If you conceived via IVF, use the EDD your fertility clinic gave you.
Is this a substitute for prenatal care?
Absolutely not. A calculated due date is a rough estimate and cannot replace proper prenatal care. Confirm your pregnancy with a qualified healthcare provider, get an early ultrasound for accurate dating, attend regular antenatal appointments, and follow medical advice about nutrition, tests, and lifestyle. This tool is for general information only.