You have a positive test, or you are about to see a doctor, and you want one thing: a due date you can plan around, and a clear idea of what "12 weeks" or "the second trimester" actually means. This guide explains where the date comes from, how to adjust it for your own cycle, why a scan can move it, and what to bring to your first appointment. It is general information, not medical advice. Your doctor or midwife decides your date.
If you only need the number right now, put the first day of your last period into the due date calculator and come back to read what it means.
Why pregnancy "starts" before you conceived
Doctors count pregnancy from the first day of your last menstrual period (LMP), not from the day of conception. Conception happens about two weeks later, so by the time a period is due you are already about four weeks pregnant by this count. The standard length is 40 weeks, or 280 days, counted from that day. The NHS calculator works the same way: you enter the first day of your last period and it estimates the due date.
The simple version of the sum, known as Naegele's rule, is: first day of last period + 280 days. Example (illustrative): the period began on 10 January 2026, so the due date is 17 October 2026. A shortcut you may hear is "subtract three months and add seven days", which gives the same answer here but can be a day out, because months have different lengths. For a period starting on 11 July, counting 280 days gives 17 April, while the shortcut gives 18 April. When in doubt, count days.
Two honest limits. First, 280 days is a convention, not a measurement of your pregnancy: a published study of the natural variation in pregnancy length found that, measured from the LMP, the average was 285 days with a standard deviation of 14 days, and it cites that only 4% of women deliver on day 280. Other papers describe the LMP-based figure as 279 days. Second, "40 weeks" is the middle of a range, not a deadline: the NHS says pregnancy normally lasts from 37 to 42 weeks from the first day of your last period.
Adjusting for your own cycle length
The 280-day rule quietly assumes a 28-day cycle, with ovulation around day 14. If your cycles are longer or shorter, ovulation, and so conception, happens later or earlier than the rule assumes. The NHS calculator therefore asks for your cycle length and tells you to adjust it if your cycle is not 28 days. The usual adjustment is to move the date by the difference from 28 days:
- 24-day cycle: due date 4 days earlier than the plain sum.
- 28-day cycle: no change.
- 32-day cycle: 4 days later (10 January becomes 21 October).
- 35-day cycle: 7 days later.
This adjustment is a convention, built on the assumption that the second half of your cycle is about 14 days long. It works well for regular cycles and badly for irregular ones. If your cycle length changes by a week or more from month to month, treat the adjusted date as a rough guide and expect the scan to settle it. The due date calculator has a cycle-length field and also draws the whole 40 weeks as a timeline, so you can slide to any week and see its dates.
If you know the conception date
If you used an ovulation test, tracked your cycle closely, or had a single known occasion, you can count from conception instead. Conception happens about two weeks after the LMP, so the length is 266 days (280 minus 14). Conceiving on 24 January 2026 gives 17 October 2026, the same date as the 10 January example above. The calculator offers this as a second method. The method assumes ovulation and fertilisation happened on the day you chose, which is the weak point: sperm can survive for days in the body, so the day of intercourse is not always the day of conception. Treat it as an estimate with a margin of a few days.
If you had fertility treatment, the clinic will give you a date based on the treatment dates. Use theirs; this guide does not cover those calculations.
Why a scan can change your date
Many women remember their last period well. Many do not, or have irregular cycles, or stopped hormonal contraception shortly before. A study comparing methods noted that LMP dating is less reliable because of poor recall, irregular cycles and prior hormonal contraception. In that study an early ultrasound gave less variable estimates of the delivery date than the LMP (a standard deviation of 8 days against 18), and its estimate fell within 14 days of the actual delivery date about 88% of the time, against roughly 66% for the LMP.
That is why the NHS offers a dating scan, usually at 11 to 14 weeks, which checks when your baby is due, along with the baby's development and whether you are expecting more than one baby. The NHS due date page also notes the 12-week scan can give a more accurate estimate of how many weeks pregnant you are. Professional guidelines decide when the scan date replaces the LMP date, based on when the scan was done and how far apart the two dates are. This guide does not quote the thresholds, so ask your clinic what it uses.
Once your doctor has settled a date, keep using that one. Using the calculator to second-guess it will only confuse you.
What the weeks and trimesters mean
Weeks of pregnancy are counted from the LMP, so they run about two weeks ahead of the baby's real age. "Twelve weeks pregnant" is roughly ten weeks after conception.
Trimesters are the three blocks of about 13 weeks. Services draw the lines slightly differently, which causes real confusion: the NICHD pregnancy page lists weeks 1 to 12, 13 to 28 and 29 to 40, while the calculator on this site ends the first trimester at 13 weeks 6 days, runs the second from 14 weeks 0 days to 27 weeks 6 days and starts the third at 28 weeks. The difference is a few days at each boundary, so for planning it does not matter. When a letter or appointment says "12 weeks", it means 12 weeks 0 days to 12 weeks 6 days.
"Term" now has four names
Under the older view, anything from 37 to 42 weeks counted as full term. The American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine recommended splitting it, after NICHD research showed that babies born at 37 or 38 weeks had poorer outcomes than those born at 39 weeks or later. As the NICHD pregnancy page lists them:
- Preterm: before 37 weeks.
- Early term: 37 weeks to 38 weeks 6 days.
- Full term: 39 weeks to 40 weeks 6 days.
- Late term: 41 weeks to 41 weeks 6 days.
- Post term: 42 weeks or later.
So your due date at 40 weeks sits in the middle of "full term". Expect your doctor to talk in these terms, especially near the end of pregnancy. These names describe when a baby is born, and they do not tell you what to do: if your doctor discusses early delivery or going past your date, ask them to explain the reasons for your case.
What to take and ask at the first visit
In the NHS system the first "booking" appointment is usually at 8 to 12 weeks and takes about an hour. The midwife asks for the date of your last period, previous pregnancies, health conditions, and checks your blood pressure, a urine sample and your blood group. That is the NHS schedule, so use it as a checklist, not a timetable, and ask your doctor for your own plan.
Bring or write down:
- The first day of your last period (a photo of a calendar or a period-tracker screenshot is fine) and your usual cycle length.
- Whether your cycles are regular, and whether you used hormonal contraception in the last few months.
- Any ovulation test or intercourse dates, if you tracked them.
- Your medicines, allergies and past conditions, and the dates of earlier pregnancies.
Questions worth asking (suggestions, not a script):
- Which date will you use as my due date, and will a scan confirm it?
- When is my dating scan, and what does it check?
- What screening tests are offered, and which are optional?
- Who do I call, and when, if I am worried between visits?
Keep the answers somewhere you can find them. A single place for your dates, scan reports and medicines saves a lot of repeating; the guide to keeping your medical file and emergency information ready covers what to put in it.
If your situation is not typical
- You do not remember your last period. Tell the doctor straight away; they can date the pregnancy by scan. The NHS suggests speaking to a midwife or GP if you do not know the date.
- Your cycles are irregular. Use the calculator only to get a rough week count and ask for an early scan.
- You stopped contraception recently. Say so; it affects how reliable the LMP date is.
- Your date has passed. Pregnancy lasting up to 42 weeks is normal in the NHS description, but contact your doctor or maternity unit to ask about the plan rather than waiting at home.
The next step
- Open the due date calculator, enter your last period and cycle length, and note the date, the weeks and the trimester.
- Write down the three facts your doctor will ask for: the first day of your last period, your cycle length, and any conception or ovulation-test dates.
- Book your first appointment: see how to book a doctor online in Egypt without calling five clinics.
If you want everything in one place, Softwiro's MyClinic lets patients search for a doctor by speciality and location and book without a phone call, and holds a medical file with their history. For a demo, contact us.
Sources
- NHS: due date calculator: enter the first day of the last period, adjust for a cycle that is not 28 days, pregnancy normally lasts 37 to 42 weeks, the 12-week scan can estimate more accurately.
- Length of human pregnancy and contributors to its natural variation: 280 days is the convention; only 4% deliver on day 280; mean length from LMP 285 days.
- Reliability of LMP recall, an early ultrasound and a smartphone app in predicting date of delivery: LMP less reliable because of recall, irregular cycles and hormonal contraception; early ultrasound less variable.
- NHS: your antenatal appointments: booking appointment at 8 to 12 weeks and what is asked; dating scan at 11 to 14 weeks.
- NICHD: pregnancy (conditions and stages) and Redefining the term: trimester ranges and the early, full, late and post term categories, and why they were introduced.