Pregnancy Due Date Calculator

How Pregnancy Due Dates Are Calculated — Methods, Accuracy and What the EDD Really Means

Your estimated due date (EDD) is one of the most significant numbers in pregnancy, yet it is widely misunderstood. Despite being called a "due date," it is not a deadline but an estimate at the centre of a statistical distribution. Understanding how the EDD is calculated, why it is an approximation rather than a prediction, and how different calculation methods compare helps you approach your pregnancy timeline with realistic expectations and confidence.

Naegele's Rule — the LMP method

The most common method for calculating a due date is Naegele's Rule, named after the German obstetrician Franz Karl Naegele who described it in the 19th century. The formula is: EDD = first day of LMP + 280 days (40 weeks). Alternatively, Naegele's original formula adds 7 days to the LMP, subtracts 3 months and adds 1 year — producing the same result. The 280-day figure assumes a regular 28-day cycle with ovulation occurring on day 14. If your cycle is longer or shorter, the standard adjustment is to add or subtract the difference from the baseline 28-day estimate. For example, a 35-day cycle shifts the EDD 7 days later; a 21-day cycle shifts it 7 days earlier. This calculator applies that adjustment automatically when you enter your cycle length.

Conception and ovulation date method

If you know the date conception occurred — for example, through confirmed ovulation tracking, a positive ovulation predictor kit, or timed intercourse — you can calculate the EDD by adding 266 days (38 weeks) from the conception date. This is exactly the same as Naegele's Rule, simply starting from a different reference point. The 266-day figure reflects the average time from fertilisation to delivery. Because gestational age is measured from LMP rather than conception, your gestational age at any point in the pregnancy will be approximately 14 days more than your fetal age (the actual age of the embryo from fertilisation).

Ultrasound dating — the most accurate method

Early first-trimester ultrasounds using crown-rump length (CRL) measurement are generally considered the most accurate method for pregnancy dating, particularly for women with irregular cycles or uncertain LMP dates. CRL measurements taken before 14 weeks have a margin of error of approximately 3–5 days. The American College of Obstetricians and Gynecologists (ACOG) recommends that the due date established by a first-trimester ultrasound takes precedence over LMP-based dating if the two differ by more than 7 days in the first trimester. After 22 weeks, ultrasound dating becomes significantly less reliable because individual variation in fetal size becomes substantial.

IVF due date calculation

For pregnancies achieved through in-vitro fertilisation, the due date calculation is more precise because the exact date of fertilisation is known. The formula differs based on the developmental stage of the embryo at transfer. A Day 5 blastocyst transfer adds 261 days to the transfer date (266 days minus 5 days of development already completed). A Day 3 embryo transfer adds 263 days to the transfer date. Day 6 blastocyst transfers add 260 days. Because the timing of fertilisation is precisely documented in IVF, these calculations are generally more accurate than LMP-based methods for natural conception.

What gestational age really means

Gestational age is the standard clinical measure of pregnancy duration, counted from the first day of your last menstrual period rather than from conception. This convention exists because the LMP is a date most women can reliably identify, whereas the exact date of ovulation or conception is usually unknown in natural pregnancies. The practical consequence is that when you are described as "four weeks pregnant," the embryo itself is actually approximately two weeks old — gestational age always runs about two weeks ahead of fetal age. All standard pregnancy milestones, trimester boundaries, and appointment timing are based on gestational age.

The full-term window — why the EDD is an estimate

Research consistently shows that only 4–5% of babies are born on their exact estimated due date. About 60% of births occur within one week of the EDD, and about 90% within two weeks. Full-term pregnancy is officially defined as 39 weeks 0 days to 40 weeks 6 days. Early term (37 to 38 weeks 6 days) and late term (41 to 41 weeks 6 days) are both considered within the normal range, though outcomes are generally best at 39–40 weeks. Babies born before 37 weeks are classified as preterm. The period of viability — when survival outside the womb becomes possible with intensive neonatal care — begins at approximately 22–24 weeks, though the risk of complications decreases significantly with each additional week of gestation.

Key prenatal appointments and screening windows

Understanding when to schedule key appointments is one of the most practical uses of a due date calculator. Your first prenatal visit is typically scheduled at 8–10 weeks, when the pregnancy is confirmed via ultrasound and baseline blood tests are taken. First-trimester screening — including nuchal translucency ultrasound and NIPT (non-invasive prenatal testing) — occurs between weeks 10 and 13. The anatomy scan (detailed structural ultrasound) is performed at 18–22 weeks to assess fetal organ development, placenta position and amniotic fluid. The glucose tolerance test for gestational diabetes screening takes place at 24–28 weeks. Group B streptococcus (GBS) screening is performed at 36–37 weeks. From 36 weeks onwards, prenatal visits become weekly as delivery approaches.

Frequently asked questions