How it works
What this calculator does
A pregnancy due date calculator tells you two things: the estimated date your baby is likely to be born (the EDD), and which week of pregnancy you are currently in. It works from three possible inputs — first day of your last menstrual period (LMP), a known conception/ovulation date, or an ultrasound dating measurement — and returns a date roughly 40 weeks after LMP or 38 weeks after conception.
NHS midwives and obstetricians use the same approach, then refine the estimate with a dating scan between 10 weeks and 14 weeks + 6 days. Crown-rump length (CRL) measurements at this stage predict EDD to within about five days either way, which is why hospitals trust scan dates over calculator dates in most cases.
Once you have an EDD, the pregnancy weight gain calculator maps the healthy trajectory across trimesters and the water intake calculator covers hydration targets that shift during pregnancy.
Naegele's rule explained
The most widely used formula, devised by German obstetrician Franz Naegele in 1812, is: EDD = LMP + 1 year − 3 months + 7 days. In practice this is simply LMP + 280 days (40 weeks).
The rule assumes a classic 28-day menstrual cycle, ovulation on day 14, and conception on that day. Real cycles vary — which is why a 35-day-cycle person will appear "overdue" by Naegele even when pregnancy is exactly on track. Modern NHS guidance corrects for cycle length where it's known, and always uses scan dating when there is any mismatch over 7 days.
Three ways to date a pregnancy
Every pregnancy can be dated in more than one way. Strengths and limits of each.
By last menstrual period
First day of your last period, plus 280 days. Easiest and most widely used. Works well for regular 28-day cycles. Less reliable if cycles are irregular, if you're on hormonal contraception in the months leading up, or if you experienced early-pregnancy bleeding that was mistaken for a period.
By conception or ovulation date
Add 266 days (38 weeks) to the conception date. Useful for IVF patients, for anyone tracking ovulation with LH strips, or for women using fertility apps. IVF dating is the most accurate of all because the egg-retrieval date is exact to the day.
By ultrasound dating scan
The NHS offers a dating scan at 10–14 weeks. Crown-rump length correlates tightly with gestational age up to about 13 weeks + 6 days. After that, biparietal diameter and femur length take over but are slightly less accurate. When LMP and scan disagree by more than 7 days in the first trimester, the scan wins.
What "40 weeks" really means
Pregnancy is counted in weeks from LMP, not weeks from conception. Because ovulation is typically two weeks after LMP, a "40-week" pregnancy actually involves 38 weeks of foetal development plus 2 weeks of pre-conception counting. This is the international convention used by WHO, NHS, NICE, the Royal College of Obstetricians and Gynaecologists (RCOG) and essentially every other medical system.
The 40-week figure is an average, not a target. Full-term runs from 37 weeks (early term) to 42 weeks (late term). Only 4 % of babies are born on their exact EDD. Around 70 % arrive within 10 days either side of it, and 90 % within two weeks.
Trimester boundaries and key milestones
The three trimesters are more than a marketing concept — they correspond to meaningful developmental shifts.
| Weeks | Trimester | Key developments | NHS touchpoint |
|---|---|---|---|
| 0–12 | First | Fertilisation, embryo formation, major organs start; risk of miscarriage highest | Booking appointment (by 10 weeks), dating scan (10–14 weeks) |
| 13–27 | Second | Anomaly scan, foetal movement starts (16–24 weeks), most comfortable phase | Anomaly scan (18–21 weeks), glucose tolerance if indicated |
| 28–40 | Third | Rapid weight gain, lung maturation, head engagement | 28, 34, 36, 38, 40, 41 weeks: growth and blood pressure checks |
How your EDD is adjusted on the NHS
UK maternity services follow a "scan date prevails" approach in most cases.
- Dating scan (10–14 weeks) measures CRL to calculate gestational age.
- If LMP and scan agree within 7 days, your EDD stays based on LMP.
- If they disagree by more than 7 days, your EDD is reset to the scan date.
- Your notes (red book in some trusts, Badger Notes app in others) show both dates and flag which one is used for decisions like induction timing.
- Anomaly scan (18–21 weeks) is about structural development, not dating — but if CRL was missed, measurements here can still correct dating.
- IVF dates always override everything else when available.
Cycle length adjustments
Naegele's rule assumes a 28-day cycle. If yours is regularly shorter or longer, you can approximate by adding (cycle length − 28) days to the EDD produced by Naegele. A person with a 35-day cycle therefore has an EDD 7 days later than Naegele suggests. The adjustment isn't perfect — ovulation timing varies even in regular cycles — but it narrows the gap that the dating scan would otherwise correct.
For very irregular cycles, scan dating is essential. Without a reliable LMP, calculator estimates may be off by 2–3 weeks, which affects timing of every antenatal test that has a week-specific window.
Maternity leave and pay in the UK
Your EDD sets the clock for employment rights and benefits.
- Statutory Maternity Pay (SMP) — 39 weeks. 90 % of average weekly earnings for the first 6 weeks, then the lower of 90 % or the SMP flat rate (£184.03/week in 2024/25).
- Earliest start — 11 weeks before the expected week of childbirth (EWC).
- MatB1 form — issued by your midwife from 20 weeks onwards; give to your employer with your notice.
- Shared Parental Leave — up to 50 weeks shared with the other parent after the first 2 compulsory weeks.
- Keeping In Touch (KIT) days — 10 paid days during maternity leave without ending it.
Antenatal appointments schedule
NHS guidance recommends the following touchpoints for a low-risk first pregnancy. Your EDD anchors them all.
| Week | Appointment | Typical content |
|---|---|---|
| 8–10 | Booking | Full medical history, blood tests, lifestyle advice |
| 10–14 | Dating scan | EDD confirmation, nuchal translucency screening |
| 16 | Midwife check | Blood test results review, blood pressure, urine test |
| 18–21 | Anomaly scan | Detailed structural scan |
| 25 | Midwife | Fundal height, blood pressure (first-time mothers only) |
| 28 | Midwife | Blood tests, anti-D if Rh-negative, glucose screen if indicated |
| 31, 34, 36, 38, 40, 41 | Midwife | Fundal height, presentation, induction discussion at 41 weeks |
What happens if you go past 40 weeks
Going past your EDD is common and not automatically a problem. NHS practice typically offers:
- Membrane sweep at 40–41 weeks to encourage labour naturally.
- Induction of labour offered between 41 and 42 weeks because risks rise modestly after 42 weeks.
- Increased monitoring — electronic foetal monitoring and amniotic fluid assessment if induction is declined.
- Shared decision-making with the midwifery team; induction is an offer, not a mandate.
Lifestyle signposts by trimester
Knowing roughly which week you are in helps you plan work, exercise and travel. A quick reference.
First trimester (0–12 weeks)
Often the most physically demanding. Nausea, fatigue, breast tenderness and food aversions peak between 6–12 weeks. Many women continue normal exercise (walking, swimming, yoga adapted for pregnancy). Avoid contact sports and scuba diving. Common travel insurance covers trips up to 28 weeks; some airlines restrict flying after 28.
Second trimester (13–27 weeks)
Usually the most comfortable. Energy returns, nausea fades, bump becomes visible. Most elective travel is done here. This is the period to tour maternity units, choose home- versus hospital-birth preference and start antenatal classes such as NHS Parent Education or NCT courses.
Third trimester (28–40 weeks)
Rapid foetal growth; baby's head typically engages after 36 weeks. Braxton Hicks contractions become more common. The NHS "birth plan" is usually written around 32–34 weeks. Hospital bag should be packed by 36 weeks and the childcare logistics sorted.
Brazilians having a baby on the NHS
If you are a Brazilian national with leave to remain in the UK (Skilled Worker, Youth Mobility, Family, Student or Settled status), you have full NHS maternity care. Key points to know:
- Register with a GP as soon as you know you're pregnant; they refer you to the local maternity team.
- The booking appointment (8–10 weeks) collects history including any previous pregnancies in Brazil — bring translated records if you have them.
- All scans, blood tests and midwife appointments are free at the point of use.
- Immigration Health Surcharge is already paid with your visa; no additional charges for maternity.
- If your baby is born in the UK, they are not automatically British; check nationality rules on GOV.UK.
- Brazilian consulates (London, Manchester) register the birth for a CRNM (Certidão de Registro de Nascimento de Brasileiro).
Dads, partners and non-gestational parents
The EDD matters for the other parent's employment rights too. UK Statutory Paternity Pay runs two consecutive weeks within 56 days of birth; Shared Parental Leave splits up to 50 weeks between parents. Same-sex couples and non-gestational parents have equivalent rights, and adoption leave mirrors the maternity framework for adopting parents.
How this calculator handles inputs
The calculator validates dates, calculates from LMP or conception/IVF transfer, and applies a cycle-length correction when you enter one. All arithmetic runs in your browser; we do not store any personal health data. For clinical dating, always rely on your midwife or obstetrician's assessment. See our editorial policy and corrections policy for how we review healthcare content.