Due Date Calculator

How to use the Due Date Calculator

Choose a calculation method and enter the relevant date to see the due date and full timeline.

1. Choose a calculation methodSelect LMP (most common), conception date or IVF transfer date. Furthermore, each method calculates the estimated due date based on standard clinical conventions.
2. Enter the relevant dateFor LMP mode, enter the first day of your last period. For conception, enter the estimated date of ovulation and fertilisation. For IVF, enter the embryo transfer date.
3. Adjust cycle length if neededFor cycles shorter or longer than 28 days, enter your actual average. Furthermore, the tool adjusts the due date by the difference from 28 days.
4. Click Calculate due dateThe due date appears in full along with gestational age today and the three trimester date ranges.
5. Note the gestational ageThe current gestational age in weeks and days tells you exactly where you are in the pregnancy. Moreover, this figure is what your healthcare provider uses at appointments.

Options and variants explained

Due date calculation depends on the method used.

The formula explained

Naegele's rule was described by German obstetrician Franz Naegele in 1812 and remains the standard method globally. The 280-day period equals 40 gestational weeks counted from the LMP — which is typically 2 weeks before ovulation. Consequently, the actual time from conception to birth is approximately 266 days (38 weeks), not 280.

Worked example: LMP 1 January, 28-day cycle

Due date: 1 January + 280 days = 7 October. Gestational age at a 20-week scan (approximately 20 May): 20 weeks 0 days. Second trimester: 1 April to 30 June. Third trimester: 1 July to due date.

With a 30-day cycle instead: add 2 days to the standard 280, giving a due date of 9 October. Furthermore, the difference is small but can matter near clinical threshold dates such as 24 weeks (viability) or 37 weeks (term).

Understanding gestational age

Gestational age is always counted from the first day of the LMP, even in IVF pregnancies where the exact conception date is known. Consequently, a pregnancy is described as 8+3 (8 weeks and 3 days) in clinical notes — always from LMP, not from conception. This convention is maintained universally for consistency in clinical reporting.

What is the expected due date?

The estimated due date (EDD) is the date at which a pregnancy is expected to reach 40 weeks of gestation. It is an estimate based on statistical averages rather than a precise prediction. Furthermore, only about 4% of babies are born exactly on the EDD, and 80% are born within two weeks either side.

Due date accuracy depends on the quality of the dating input. Early ultrasound (before 14 weeks) is the most accurate method and takes precedence over LMP dating when the two differ. Moreover, LMP dating assumes a 28-day cycle and ovulation on day 14 — assumptions that do not hold for everyone.

Gestational age is expressed in weeks and days rather than months because fetal development milestones are defined by week. The second trimester begins at 13 weeks and the third at 26 weeks. Furthermore, 37 weeks is considered early term and 40–41 weeks is full term. Post-term begins at 42 weeks.

Why accurate due date calculation matters

Clinical decisions throughout pregnancy depend on gestational age. Screening tests, anomaly scans and growth monitoring all have recommended gestational age windows. Furthermore, antenatal care appointments are timed relative to gestational age, not calendar dates.

The viability threshold at approximately 23–24 weeks is a critical milestone. Interventions and intensive care decisions in preterm birth are informed by gestational age. Moreover, birth before 28 weeks, before 32 weeks, before 37 weeks and after 42 weeks each carry distinct clinical implications.

For birth planning, the due date helps organise practical preparations — leave dates, hospital bag timing and family notification. Additionally, knowing all three trimester dates helps time antenatal appointments, screening tests and preparation milestones.

Common due date misconceptions

Assuming the due date is the exact birth date leads to unnecessary anxiety when that date passes. Only 4% of babies arrive on the EDD. Furthermore, labour is considered normal between 37 and 42 weeks — the due date is simply the statistical midpoint of this window.

Using conception date when it is uncertain produces a less accurate result than LMP dating. Ovulation timing can vary by several days, and fertilisation can occur 24 hours after ovulation. Moreover, early ultrasound dating from fetal crown-rump length measurement is more accurate than either when performed before 14 weeks.

Ignoring cycle length differences skews LMP-based calculations. A 35-day cycle pushes ovulation later, so the EDD should be later. Conversely, a 24-day cycle shifts ovulation earlier and advances the EDD. Furthermore, this calculator adjusts automatically when you enter your actual cycle length.

Tips for tracking pregnancy progress

Download a pregnancy tracking app that shows week-by-week fetal development milestones. Understanding what is happening developmentally each week helps contextualise scan findings and symptoms. Moreover, knowing gestational age precisely helps identify when symptoms are within the expected range.

Write down your LMP date in multiple places — phone, calendar, medical notes. Furthermore, healthcare providers consistently ask for this date at every appointment, and having it readily available avoids confusion when discussing gestational age.

Schedule first-trimester appointments promptly. The dating scan between 11 and 14 weeks refines the EDD using fetal crown-rump length, which is more accurate than LMP calculation. Additionally, the scan screens for chromosomal conditions such as Down syndrome using nuchal translucency measurement.

Frequently asked questions

Related tools