How Many Weeks Pregnant Am I? Calculator and Guide

Counting pregnancy weeks trips up almost everyone — the calendar says nine months, but your midwife counts 40 weeks. That’s because pregnancy math starts two weeks before conception, measured from the first day of your last menstrual period. Getting that starting point right is the key to answering “how many weeks pregnant am I?” accurately.

Average pregnancy length: 40 weeks ·
Full-term range: 37-42 weeks ·
Calculation basis: First day of last period ·
Early bump possible: At 5 weeks ·
Due date sources: Johns Hopkins Medicine

Pregnancy duration figures from major UK health authorities and international standards align on the core metrics.

The pattern across UK health services shows consistent use of the 280-day calculation from last menstrual period, with regional variations in presentation but not in underlying methodology.

How do I know how many weeks pregnant I am?

The answer hinges on one input: the first day of your last menstrual period (LMP). Every major calculator — including the NHS pregnancy due date calculator — uses this date to estimate your current week and due date. Here’s how the two main calculation approaches work:

Using last menstrual period

You input the first day of your last period into an NHS calculator. The tool adds 280 days — that’s 40 weeks — to reach your estimated due date. Regional UK services like Somerset NHS Foundation Trust and NHS 111 Wales offer the same calculation. If your cycle runs longer or shorter than the standard 28 days, Tommy’s calculator lets you adjust for that.

Pregnancy test timing

Most home pregnancy tests detect hCG around 10-14 days after conception, which typically falls 3-4 weeks after your LMP. The test result tells you pregnancy has started — but to answer “how many weeks,” you still need to count forward from LMP. This is why the NHS calculator and similar tools ask for your LMP date specifically, not the test date.

The implication: skipping the LMP date means your calculator result will be inaccurate from the start.

How many weeks am I pregnant from my last period?

Subtract the first day of your LMP from today’s date, then divide by seven. Most due date calculators do this automatically. But two calculation methods exist, and they produce different week counts:

LMP-based formula

The LMP method adds 280 days (40 weeks) to the first day of your last menstrual period. According to American Pregnancy Association, this assumes a 28-day cycle with ovulation on day 14. The Government of Jersey uses this same 280-day model in its NHS-linked calculator.

Pregnancy week calculator by LMP

The pregnancy week calculator by LMP starts counting from LMP day one, even though conception hasn’t occurred yet. Tommy’s notes this means the first two weeks are counted before pregnancy technically exists. The calculation doesn’t change — 280 days from LMP — but understanding why those two weeks are included matters for interpreting your current week number.

What this means: women with irregular cycles risk miscalculating their due date by a week or more if they rely solely on the basic LMP method.

Why is it 9 months but 40 weeks?

Here’s the catch that trips up almost everyone: nine calendar months doesn’t equal 40 weeks exactly. Nine months is roughly 36-37 weeks. The 40-week standard includes those first two weeks counted before conception. NHS guidance confirms pregnancy normally lasts 37 to 42 weeks from LMP. The extra weeks beyond nine months account for the two-week pre-conception count plus the natural variation in when labor actually starts.

Weeks vs. months

The math doesn’t line up cleanly. Nine months × 4.3 weeks per month = roughly 38.7 weeks. But the clinical standard measures from LMP, which adds two weeks before ovulation. That’s why 40 weeks feels longer than nine months on the calendar — because from your actual conception, it’s closer to 38 weeks.

Flo app explanation

Popular tracking apps like Flo use the same 40-week standard because it aligns with how healthcare providers measure pregnancy. The discrepancy between “9 months” (as friends and family ask about) and “40 weeks” (as your midwife counts) is why you might feel confused about your actual progress. The takeaway: when someone asks which month you’re in, a quick conversion helps — week 12 is roughly the end of month 3.

The catch: telling friends “I’m eight months pregnant” understates your actual progress by about two weeks compared to your medical records.

Is my due date 37 or 40 weeks?

Technically, your estimated due date is 40 weeks from LMP — but your actual full-term window spans 37 to 42 weeks. Somerset NHS Foundation Trust states pregnancy lasts 37-42 weeks, which aligns with NHS guidance. Your due date is the midpoint of that range, not a deadline.

Full-term definition

Full term means your baby is developed enough for life outside the womb with minimal medical intervention. NHS and Johns Hopkins Medicine both define full-term as 37-42 weeks from LMP. Babies born before 37 weeks are preterm; those born after 42 weeks are post-term and may require monitoring or induction.

Calculating a Due Date

The standard method adds 280 days to LMP. NHS pregnancy due date calculator guidance confirms this baseline, while noting that early ultrasound at 12 weeks refines the estimate. The Patient.info calculator lets you input LMP or ultrasound dating for a more tailored result.

The implication: doctors measure your baby’s development against the 38-week fetal age, not the 40-week official count.

How long is pregnancy?

Pregnancy normally lasts 40 weeks from LMP — that’s the global clinical standard. NHS pregnancy section states pregnancy normally lasts 37 to 42 weeks from the first day of your last period. The wide range acknowledges that bodies vary and labor timing isn’t exact.

Weeks and months explained

The 40-week count breaks down into three trimesters: weeks 1-13 (first trimester), weeks 14-26 (second trimester), and weeks 27-40 (third trimester). Months map loosely to four-week blocks, but the conversion never quite lines up perfectly — which is why saying “I’m in month X” often requires a mental adjustment when switching to weeks.

How many weeks pregnant will I be on a date

To find your pregnancy week on any future date, count the days between your LMP and that date, then divide by seven. Online calculators like NHS and Tommy’s display both your current pregnancy week and your estimated due date simultaneously, making it easy to plan ahead. If you’re asking “how many weeks pregnant will I be on [date],” the math is straightforward: days since LMP ÷ 7.

What this means: planning major life events around your due date gives you a five-week buffer on either side to account for natural variation.

How to calculate your pregnancy week step by step

Here’s the practical workflow, grounded in how NHS and specialist calculators work:

1. Pin down your LMP. Find the first day of your last period on a calendar. This is the starting point for every due date calculator. NHS and NHS 111 Wales both require this specific date input.
2. Choose your calculator. The NHS pregnancy due date calculator offers the standard 40-week estimate. Tommy’s calculator adds a cycle-length adjustment if your cycle differs from 28 days.
3. Input your LMP and read your result. The calculator adds 280 days to your LMP date and displays your estimated due date (40 weeks from LMP). It also shows your current pregnancy week based on today’s date.
4. Schedule your midwife booking. Book by 10 weeks — Tommy’s guidance recommends this timeline so you can arrange your dating scan.
5. Confirm at your 12-week scan. NHS offers a 12-week dating scan that refines your due date based on fetal measurements. This often adjusts the LMP estimate by several days.
6. Track week by week. Once your due date is confirmed, count forward from LMP to find your exact pregnancy week on any given day. Most apps and Patient.info calculator display this automatically.

The implication: women with cycles outside the 22-44 day range should push for early ultrasound dating rather than relying on basic LMP calculation.

Pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period.

NHS (UK National Health Service) — Due Date Calculator guidance

Calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP), this standard clinical method provides a helpful baseline.

American Pregnancy Association — Pregnancy Calculator resource

Your period and ovulation are counted as the first 2 weeks of pregnancy.

Tommy’s (UK Pregnancy Charity) — Pregnancy Due Date Calculator

The implication: NHS, Tommy’s, American Pregnancy Association, and Patient.info all urge midwife consultation to confirm your estimate. For those with irregular cycles, Tommy’s provides cycle-length adjustment; Patient.info notes the LMP method may be less reliable. The 12-week ultrasound typically provides the most accurate dating and can shift the due date by several days or weeks.

For anyone trying to track their pregnancy week, the path is clear: start with your LMP, use the NHS calculator as your baseline, adjust for cycle length if needed, and confirm everything with your midwife at the booking appointment. The Government of Jersey calls the NHS calculator a “rough guide” — and that’s the honest framing. Your due date is a midpoint in a five-week window, not a deadline. What matters most is getting that first input right (your LMP) and confirming it with professional guidance after 10 weeks.