🤱 Pregnancy Week Calculator

Enter your last menstrual period (LMP) date or current gestational week to track your pregnancy progress, trimester, and key milestones.

Last Menstrual Period (LMP)

Due date is calculated as LMP + 280 days (Naegele's Rule)

What is a Pregnancy Week Calculator?

A pregnancy week calculator determines current gestational age (in weeks and days), the estimated due date (EDD), and trimester based on the first day of the last menstrual period (LMP) or a known conception date. Gestational age is the standard clinical measure of pregnancy duration — counted from the first day of the last menstrual period rather than from the actual date of conception, because LMP is a known and reliable reference point while conception date is typically unknown or only approximately known.

A full-term human pregnancy is 40 weeks (280 days) from LMP — which means conception occurred approximately at week 2 and the embryo/foetus has been developing for 38 weeks at the time of birth. Gestational age at birth can range from 37–42 weeks and be considered full-term (with late preterm 34–36 weeks, preterm below 34 weeks, and post-term above 42 weeks). Each obstetric appointment references gestational age in weeks and days — understanding where you are in the gestational timeline helps make sense of milestone tests, scans, and developmental stages communicated by healthcare providers.

The due date (EDD) is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the LMP, adjusted for cycle length differences from the standard 28-day cycle. Only approximately 5% of babies are born on their exact due date — 80% of births occur within 2 weeks before or after the EDD. The due date is best understood as the centre of an expected birth window rather than a fixed target date. For women with irregular cycles or who conceived via IVF, gestational age is more precisely determined by ultrasound crown-rump length measurement at the first trimester scan.

Pregnancy Trimesters

First Trimester Weeks 1–12

Major organ development. Morning sickness common. First heartbeat detectable around week 6.

Second Trimester Weeks 13–26

Baby starts moving (quickening). Anatomy scan around week 20. Most comfortable trimester for many.

Third Trimester Weeks 27–40

Rapid growth. Baby gains most weight. Lungs mature. Full term at 39–40 weeks.

How the Pregnancy Week Calculator Works

Formula, assumptions, and calculation steps for this daily life tool.

Methodology

Daily-life calculators turn common date, time, budget, and household inputs into quick practical estimates.

Calculation Steps

Enter the everyday values requested by the form.
Normalize dates, times, currency, or quantities as needed.
Apply the simple arithmetic or calendar rule.
Show the result in a format that is easy to act on.

Assumptions and Limits

Local rules, time zones, and rounding choices may affect real-world results.
The calculator uses the values entered and does not verify external schedules.
Use results as a planning aid.

Frequently Asked Questions

How is the due date calculated?

The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. Only about 5% of babies are born on their exact due date.

What is gestational age vs. fetal age?

Gestational age is counted from the first day of your last menstrual period (LMP) and is the standard medical measure. Fetal age (or conceptional age) counts from conception, which is typically 2 weeks after LMP. Doctors and ultrasounds always use gestational age.

When can I hear the heartbeat?

A heartbeat is typically detectable via vaginal ultrasound at around 6 weeks gestational age. With a standard abdominal ultrasound, it is usually detectable at 7–8 weeks. A Doppler device can detect it from about 10–12 weeks onward.

What happens at the 20-week anatomy scan?

The mid-pregnancy anatomy scan (typically at 18–22 weeks) checks the development of all major organs, the placenta position, amniotic fluid levels, and can reveal the baby's sex if desired. It is the most comprehensive ultrasound of pregnancy.

What does full term mean?

Full term is 39–40 weeks. Early term is 37–38 weeks, late term is 41 weeks, and post-term is 42+ weeks. Most OBs prefer not to let pregnancy go beyond 42 weeks and will discuss induction options at that point.

Real-World Applications

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Prenatal Appointment Scheduling

Knowing exact gestational age determines when specific prenatal tests are scheduled — NIPT/combined screening is offered at 10–13 weeks; anomaly ultrasound at 18–20 weeks; glucose tolerance test at 24–28 weeks.

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Birth Planning & Leave Timing

Parents and HR departments use the EDD to calculate statutory maternity/paternity leave start dates — UK SMP starts 11 weeks before EDD; US FMLA leave planning requires knowing the expected birth week.

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Neonatal Risk Stratification

Gestational age at birth determines neonatal care pathway — infants born before 34 weeks are admitted to a NICU; 34–36+6 weeks (late preterm) require special monitoring; 37–41 weeks are considered full-term.

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Medication Safety Assessment

Healthcare providers reference gestational age when assessing medication safety — some drugs are teratogenic only in the first trimester (organogenesis), while others pose risks in the third trimester (fetal circulation effects).

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Maternity Benefits & Entitlements

Many maternity benefit eligibility and payment calculations reference the EDD or gestational age — the UK 15-week qualifying window, SMP payment start date, and Maternity Allowance qualifying rules all depend on knowing exact gestational age.

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Antenatal Class Scheduling

Antenatal education classes are timed to gestational age — most NCT and NHS birth preparation courses are designed to be completed by week 36, requiring knowledge of current gestation to enrol at the right time.

Common Mistakes

Counting from conception date instead of LMP

Gestational age is counted from the first day of the last menstrual period — not from the date of conception or intercourse. Conception typically occurs around week 2 of gestational age. Using the conception date as day 0 understates gestational age by approximately 2 weeks.

Using the end of the last period instead of the first day

The LMP anchor date is the first day of the last period — not the last day. A period that runs from June 1–5 has an LMP anchor of June 1. Using June 5 shifts the EDD by 4 days, which can affect the timing of first-trimester screening windows.

Not updating the EDD after an ultrasound dating scan

If a first-trimester dating ultrasound gives an EDD that differs from the LMP-based estimate by more than 7 days (ACOG threshold), the ultrasound-based date should be used as the official EDD. Many women continue using the LMP-based date when the scan-confirmed date is more accurate.

Treating the due date as a specific deadline

Only ~5% of babies are born on their exact EDD. Normal term birth spans 37–42 weeks (39–41 weeks is optimal). Approaching the due date as a fixed deadline creates unnecessary anxiety — it is the midpoint of a 4–5 week expected delivery window.

Confusing gestational age with embryonic (fetal) age

Gestational age (from LMP) = embryonic/fetal age (from fertilisation) + ~2 weeks. A sonographer reporting a fetal age of 18 weeks means gestational age of ~20 weeks. Clinicians always use gestational age; embryonic age is used in embryology and IVF contexts.

Key Prenatal Milestones by Gestational Week

Week	Clinical Milestone / Screening
6–8 weeks	First booking appointment; heartbeat detectable by transvaginal scan
10–13 weeks	NIPT / combined first trimester screening; nuchal translucency scan
18–20 weeks	Anomaly (anatomy) scan — checks structural development
24–28 weeks	Glucose tolerance test (GDM screening); whooping cough vaccination
28 weeks	Anti-D injection for Rh-negative mothers; third trimester begins
36–40 weeks	Group B Strep swab; weekly monitoring if post-term; birth preparation

References

ACOG. "Methods for Estimating the Due Date." Committee Opinion No. 700. American College of Obstetricians and Gynecologists, 2017.
NICE. Antenatal Care Guideline NG201. National Institute for Health and Care Excellence, 2021.
Naegele, F.K. Lehrbuch der Geburtshülfe. 1812.
Hadlock, F.P. et al. "Sonographic Estimation of Gestational Age." Radiology, 1992.
WHO. WHO Recommendations on Antenatal Care for a Positive Pregnancy Experience. World Health Organization, 2016.