How Many Weeks Pregnant Am I? Calculate Weeks

That moment when you first try to figure out exactly how far along you are can feel oddly like solving a puzzle with half the pieces missing. Whether you’re using a calendar, an app, or just counting back from a due date, the answer to “how many weeks pregnant am I?” depends entirely on which method you use — and each method has its own level of precision. This guide breaks down the four main ways to calculate gestational age, from your last period to IVF transfer, so you can get the clearest estimate for your situation.

Average pregnancy length: 40 weeks (280 days) from last menstrual period ·
Full-term range: 37 to 42 weeks ·
First day of last period method: most common way to estimate weeks ·
Due date calculation formula: add 280 days to LMP date ·
IVF pregnancy dating: based on embryo transfer date ·
Ultrasound dating accuracy: within 5-7 days in first trimester

Six key numbers summarize what every pregnancy calculator relies on:

How many weeks am I pregnant from my last period?

Step 1: Find the first day of your last menstrual period

The standard LMP method estimates due date by adding 280 days, or 40 weeks, to the first day of the last menstrual period, according to the American Pregnancy Association. This approach assumes a regular 28-day cycle and ovulation around day 14.

Step 2: Count the weeks from that date to today

Once you have your LMP date, count forward week by week to today’s date. A common rule of thumb is Naegele’s rule, which adds 40 weeks to the LMP to find the due date — explained by InVia Fertility.

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

– HSE (Health Service Executive Ireland) guidance

In pregnancy dating, conception-based timing is approximately 266 days, or 38 weeks, from fertilization to birth (Perinatology.com).

Step 3: Use a pregnancy calculator or manual math

The NHS and HSE provide free calculators that require your LMP date. The math is straightforward: divide total days since LMP by 7 to get weeks pregnant. For example, 63 days since LMP equals 9 weeks exactly.

How to use a pregnancy week calculator effectively

Most online calculators ask for your LMP or due date. Enter the first day of your last period for LMP-based results, or the due date if known. The calculator will display your current week and estimated due date. For reliability, use tools from trusted health services such as the NHS or HSE.

The implication: If your cycle is irregular, an early ultrasound is the most reliable way to confirm your due date.

How many weeks am I pregnant from my due date?

What the due date represents

A due date is simply an estimate of when 40 weeks from LMP will be reached. It is not a guarantee — most babies arrive between 37 and 42 weeks. The London Pregnancy due date calculator uses similar logic but adjusts for certain transfer scenarios.

Working backward from the due date

If you only know your due date, subtract the number of weeks remaining from 40. For instance, if your due date is 8 weeks from today, you are approximately 32 weeks pregnant. This method is a quick estimation, not a substitute for LMP or ultrasound dating.

Why the due date may differ from LMP-based calculation

Due dates are often shifted after a first-trimester ultrasound. According to the Perinatology calculator from Perinatology.com, if the ultrasound crown-rump length differs from the LMP estimate by more than 7 days, the due date is typically adjusted. This means working backward from a due date may yield a different week count than counting forward from LMP.

The pattern: Relying on a due date alone introduces uncertainty; always cross-check with LMP or ultrasound for accuracy.

How many weeks pregnant am I if I had IVF?

IVF pregnancy dating basics

IVF pregnancy dating commonly uses the egg retrieval date as a proxy for ovulation or fertilization timing, notes InVia Fertility. One IVF dating shortcut is to subtract 14 days from the date of ovulation to create a corrected LMP — after that, gestational age is calculated from the corrected LMP just as in non-IVF pregnancies (InVia Fertility).

Calculating weeks after a fresh embryo transfer

For a fresh embryo transfer, SingleCare states that the calculation uses the egg retrieval date and adds 38 weeks, or 266 days, to estimate the due date (SingleCare). For a day-3 transfer, subtract 3 days from the 266-day calculation; for a day-5 transfer, subtract 5 days (SingleCare). The American Pregnancy Association offers a similar method: from a day-3 embryo transfer, add 263 days; from a day-5 blastocyst transfer, add 261 days.

Calculating weeks after a frozen embryo transfer

Frozen embryo transfer works on the same logic based on embryo age at transfer. For day-5 blastocyst transfer, the London Pregnancy calculator handles day-3 and day-5 transfers by subtracting embryo age from transfer date and then adding 266 days. In practice, for an IVF pregnancy, the menstrual age is described as 14 days more than the actual embryo age (InVia Fertility).

The implication: IVF dating is the gold standard for precision; rely on your clinic’s calculation over any generic tool.

How many months pregnant am I by week?

Weeks to months conversion chart

The relationship between weeks and months is not exact in calendar terms. Here is the standard breakdown:

• Month 1: weeks 1–4
• Month 2: weeks 5–8
• Month 3: weeks 9–13
• Month 4: weeks 14–17
• Month 5: weeks 18–22
• Month 6: weeks 23–27
• Month 7: weeks 28–31
• Month 8: weeks 32–35
• Month 9: weeks 36–40

Why months are not exact weeks

Calendar months range from 28 to 31 days, so dividing weeks into months is approximate. A full-term pregnancy is roughly 9 calendar months but 40 weeks, or 280 days. The trimesters break down as: first trimester (weeks 0–12), second trimester (weeks 13–27), and third trimester (weeks 28–40).

Using a month calculator for pregnancy

Several online tools convert weeks to months, but rounding varies. Some count month 1 as weeks 1–4, others count from 0. The safest approach is to use a calculator from a reliable source like the American Pregnancy Association that explicitly shows trimester matches.

The catch: Month-based conversions are rough estimates; always default to weeks for clinical accuracy.

Blockquotes from trusted sources

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

– HSE (Health Service Executive Ireland) guidance

Our easy-to-use pregnancy calculator is the number one most popular way to calculate your due date.

– American Pregnancy Association

IVF due date calculator uses embryo transfer date and embryo age to estimate fertilization date.

– London Pregnancy

Summary

No matter which method you use — LMP, due date, IVF, or ultrasound — the answer to “how many weeks pregnant am I?” comes with a built-in margin of uncertainty. For someone with regular cycles, LMP is a solid starting point; for those with irregular periods or who’ve done IVF, ultrasound or corrected LMP from a clinic gives the most reliable weeks estimate. For anyone pregnant today, the clearest path is to ask your provider for an ultrasound-based dating early in pregnancy — or, if you’re still calculating at home, stick with a calculator from a government health service like the American Pregnancy Association, NHS, or HSE rather than generic apps. A week’s difference can change which trimester milestones apply, so getting the number right matters for your prenatal care decisions.

For a quick and reliable estimate, you can use an accurate pregnancy calculator that factors in your last menstrual period or due date.

Frequently asked questions