Pregnancy Calculator
Due date, current week, trimester and weekly milestones.
Estimates use Naegele’s rule (LMP + 280 days), adjusted for your cycle.
Monday, November 2, 2026
an ear of corn
Taste buds are forming and the inner ear is fully developed — baby may respond to your voice.
a rutabaga — Baby is gaining fat and skin is becoming less translucent.
This pregnancy calculator estimates your due date and shows exactly how far along you are. Enter the first day of your last menstrual period (LMP) and, optionally, your average cycle length. Using Naegele's rule — LMP plus 280 days, adjusted for a cycle that is not 28 days — it works out your estimated due date, your current week and day, your trimester, and a week-by-week guide to your baby's growth, including a size comparison and a note on what is developing that week. Nothing you enter is transmitted anywhere; the whole estimate is computed on your device.
How to use the pregnancy calculator
- Enter the first day of your last menstrual period in the date field.
- Adjust the cycle-length slider if your cycle is not the standard 28 days (it is clamped to a 20–45 day range).
- Read your estimated due date and the coloured trimester badge showing your current week and day.
- Follow the 40-week timeline to see how far along you are as a proportion of the pregnancy.
- Open “Week by week” to read every milestone from week 4 through week 40.
Worked examples
LMP 26 January 2026, 28-day cycle
280 days from 26 January gives an estimated due date of 2 November 2026. Read on 13 July 2026, that places you at 24 weeks and 2 days — the second trimester, with the baby around the size of an ear of corn.
The same LMP with a 35-day cycle
A 35-day cycle means ovulation happens about a week later than the textbook day 14, so the due date shifts seven days later to 9 November 2026 and your current week drops by one. This is why the cycle-length field matters for anyone whose cycles are consistently long or short.
Working backwards from a due date
If a scan has given you a due date, subtract 280 days to get the LMP the calculator expects. A due date of 1 March 2027 corresponds to an LMP of 25 May 2026 on a 28-day cycle — enter that and the weekly timeline will line up with your scan dating.
Why 40 weeks starts before conception
The most confusing part of pregnancy dating is that week 1 happens before you are pregnant. Gestational age is counted from the first day of your last period, not from conception, because the LMP is a date most people can actually remember while the moment of conception almost never is. On a textbook 28-day cycle, ovulation lands around day 14, so conception occurs at roughly two weeks gestational age and implantation around week 3.
That means fetal age — the actual age of the embryo — runs about two weeks behind the gestational age shown here and in every clinic. A pregnancy described as “12 weeks” involves an embryo that has been developing for about 10. Both numbers are correct; they simply measure from different starting points, and gestational age is the one used universally for scheduling scans and screening.
Naegele's rule adds 280 days (40 weeks) to the LMP. The cycle adjustment in this calculator adds the difference between your cycle length and 28 days, which shifts the assumed ovulation date accordingly. A 32-day cycle adds four days; a 24-day cycle subtracts four.
How accurate is a due date?
An estimated due date is a midpoint, not a deadline. Only around 4% of babies arrive on the exact date predicted, and a term birth is anything from 37 to 42 weeks. Roughly half of births happen within a week either side of the estimate, and most of the rest within two.
LMP-based dating assumes a regular cycle and reliable recall of the date. Where cycles are irregular, where hormonal contraception was recently stopped, or where the LMP is uncertain, a first-trimester ultrasound is substantially more precise — crown-rump length measured between about 8 and 13 weeks dates a pregnancy to within a few days. Clinicians will usually revise the due date to the scan estimate when the two disagree by more than about a week.
Trimesters and what the timeline shows
Pregnancy is conventionally split into three trimesters: weeks 1–13, weeks 14–27 and weeks 28–40. The badge is colour-coded and updates as your week advances, and the 40-week bar shows your position as a proportion of the whole.
The week-by-week guide runs from week 4 to week 40 and pairs each week with a familiar size comparison and a short note on the main developmental event. Those comparisons are conventional averages published in pregnancy references, not measurements of your pregnancy — real babies vary considerably, and variation is normal.
This tool is for information and planning. It cannot detect complications, cannot confirm a pregnancy, and is not a substitute for antenatal care. Anything that worries you belongs with your midwife or doctor, not a calculator.
Frequently asked questions
How is my due date estimated?
We use Naegele's rule: 280 days (40 weeks) from the first day of your last period, adjusted by the difference between your cycle length and 28 days. It is an estimate — only about 1 in 20 babies arrive on the exact due date.
What if my cycle isn't 28 days?
Use the cycle-length slider. A longer cycle pushes the estimated due date later and a shorter cycle brings it earlier, because ovulation timing shifts accordingly. A 32-day cycle moves the estimate four days later.
Why does week 1 start before I was pregnant?
Gestational age is counted from the first day of your last period because that is a date you can identify, unlike the moment of conception. Conception happens around week 2 and implantation around week 3 on a typical cycle.
What is the difference between gestational age and fetal age?
Gestational age is measured from your LMP; fetal age is measured from conception and runs roughly two weeks behind. Clinics use gestational age, which is what this calculator shows.
How accurate is an LMP-based estimate?
It is a reliable starting point, but an early ultrasound dating scan is more precise, especially with irregular cycles. Crown-rump length measured between 8 and 13 weeks dates a pregnancy to within a few days. Always confirm with your midwife or doctor.
What does the trimester badge mean?
Pregnancy is split into three trimesters: weeks 1–13 (first), 14–27 (second) and 28–40 (third). The badge is colour-coded and updates as your week advances.
Can I enter my due date instead of my LMP?
Not directly, but you can work backwards: subtract 280 days from your due date and enter that as the LMP with a 28-day cycle. The weekly timeline will then match your scan dating.
Is this a substitute for medical advice?
No. This tool is for general information and planning only, and it cannot assess your pregnancy or detect complications. Always consult a qualified healthcare professional.