Pregnancy Calculator

How is it computed?

For the EDD we apply Naegele's rule: LMP plus 280 days (40 weeks), adjusted when the cycle runs longer or shorter than 28 days.

Gestational age is simply the time elapsed since the LMP, counted in weeks and days. As for the trimester, the 1st runs up to 13 weeks, the 2nd covers 14-27 weeks and the 3rd starts at 28 weeks.

This is a statistical estimate. Precise gestational age is only confirmed by medical exams.

Gestational age, due date and the science behind the numbers

A pregnancy due date is one of the first numbers an expectant family wants to know, but the figure shown by any calculator is the result of a careful convention agreed upon by obstetricians worldwide. This page explains what gestational age really means, how the estimated date of delivery (EDD) is derived, why ultrasound often refines that estimate, and when the value should be recalculated. The aim is to help you read your prenatal records with confidence, not to replace consultations with your obstetrician or midwife.

What gestational age is and how it is measured

Gestational age is the time elapsed since the first day of the last menstrual period (LMP), expressed in completed weeks and days. By convention, pregnancy is counted from the LMP rather than from conception because the menstrual date is easier to identify and remember than the exact day of ovulation. As a consequence, the first two weeks of gestational age technically precede fertilisation, which usually occurs around the middle of the cycle. The World Health Organization (WHO) and the Brazilian Ministry of Health use this same convention in prenatal records, ultrasound reports and the national pregnancy booklet (Caderneta da Gestante).

A term pregnancy is defined as 280 days, or 40 weeks, from the LMP. The American College of Obstetricians and Gynecologists (ACOG) further subdivides term gestation into early term (37 weeks 0 days to 38 weeks 6 days), full term (39 weeks 0 days to 40 weeks 6 days), late term (41 weeks 0 days to 41 weeks 6 days) and post-term (42 weeks 0 days and beyond). Pregnancies between 20 and 36 weeks 6 days are considered preterm, and births under 22 weeks fall outside viability in most national guidelines.

Naegele's rule explained with a worked example

The classical method behind almost every due-date calculator is Naegele's rule, named after the German obstetrician Franz Karl Naegele (1778-1851). The formula is simple: take the first day of the last menstrual period, subtract three months and add seven days (and adjust the year when the calculation crosses January). The arithmetic adds up to roughly 280 days from the LMP, which is the statistical mean length of human pregnancy in women with regular 28-day cycles.

As a worked example, imagine the LMP was on 10 June 2026. Subtracting three months gives 10 March 2026; adding seven days gives 17 March; finally, we move to the following year because pregnancy spans approximately nine months and a week, so the estimated due date is 17 March 2027. The calculator above performs this same operation automatically and also tells you the current week and day of pregnancy and the trimester you are in.

The three trimesters: fetal development milestones

Although the cut-offs vary slightly between guidelines, the most widely used division places the trimesters as follows:

• First trimester (weeks 1 to 13): implantation, formation of the neural tube, beating heart visible on ultrasound from week 6, basic organogenesis and limb buds. This is when the embryo is most sensitive to teratogens and when crown-rump length (CRL) provides the most accurate dating.
• Second trimester (weeks 14 to 27): rapid growth, recognisable facial features, sex usually identifiable on ultrasound, perception of fetal movements (quickening) between weeks 18 and 22, morphology scan typically performed between weeks 20 and 24.
• Third trimester (weeks 28 to 40+): accelerated weight gain, lung maturation with surfactant production, sleep-wake cycles, descent into the pelvis and preparation for labour. Antenatal visits become biweekly and then weekly toward term in most national protocols.

Why LMP is not always precise: the role of ultrasound

Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. In real life, cycles range widely, recall of the LMP can be inaccurate, early pregnancy bleeding can be confused with a period, and stopping hormonal contraception can delay ovulation. For these reasons, ACOG and the National Institute for Health and Care Excellence (NICE) recommend an early ultrasound to confirm or correct the EDD whenever possible.

First-trimester ultrasound dating by crown-rump length is the most accurate method available, with a margin of error of approximately five to seven days when performed between 8 weeks 6 days and 13 weeks 6 days. In the second trimester, biometric measurements such as biparietal diameter, head circumference, abdominal circumference and femur length offer an accuracy of around seven days. After 22 weeks, scans become progressively less reliable for dating because individual fetal growth patterns diverge. ACOG suggests adjusting the EDD when the ultrasound estimate differs from the LMP-based estimate by more than five days before 9 weeks, seven days between 9 and 16 weeks, ten days between 16 and 22 weeks, or fourteen days between 22 and 28 weeks.

When to recalculate: irregular cycles and late ovulation

If your cycles are longer than 28 days, Naegele's rule will tend to overestimate gestational age (i.e., suggest you are further along than you really are). Parikh's formula is a refinement that adds nine months to the LMP, subtracts 21 days, then adds the average length of your previous cycles. The calculator on this page lets you adjust the cycle length so it can apply a similar correction without changing the formula manually.

In pregnancies achieved through assisted reproduction, the conception date is known precisely and the EDD is set by counting 266 days from the date of egg retrieval or 263 days from the day-3 embryo transfer. Twin and higher-order pregnancies share the same LMP-based estimate, but most guidelines recommend planned delivery earlier than singletons because the risks of stillbirth, preeclampsia and growth restriction increase past 37-38 weeks for twins.

Frequently asked questions

• Is the EDD the day my baby will be born? No. Only about 4% of babies are born exactly on the EDD; roughly 80% arrive within ten days of it, and a healthy delivery is considered any time between 37 and 42 weeks.
• What is "corrected EDD"? The corrected (or revised) EDD is the date obtained by adjusting the original LMP-based estimate with a first-trimester ultrasound. Once it is in your records, it should be used for all subsequent decisions.
• I had spotting after the LMP — does that change my EDD? Possibly. If the spotting was actually an implantation bleed rather than a period, your true LMP may have been earlier. An ultrasound resolves the question quickly.
• Why does the calculator give a slightly different result than my doctor? Your clinician may already be using an ultrasound-corrected EDD, a different cycle length, or a +14-day rule for the day of conception in IVF cycles.

Medical disclaimer. This calculator is for educational purposes only and does not constitute medical advice. Estimated due dates, gestational age and trimester information should always be confirmed by a qualified obstetrician or midwife, ideally with a first-trimester ultrasound. If you have a high-risk pregnancy, vaginal bleeding, severe pain or any concerning symptom, contact your healthcare provider or the nearest emergency service immediately.

Authoritative references consulted: World Health Organization recommendations on antenatal care; ACOG Committee Opinion 700, "Methods for Estimating the Due Date"; NICE guideline NG201, "Antenatal care"; NHS pregnancy guide; StatPearls "Estimated Date of Delivery"; Brazilian Ministry of Health, Caderneta da Gestante (electronic edition, 2022).