How Many Weeks Pregnant Am I – Accurate Calculator & Methods

Many expectant parents ask the same question early on: how many weeks pregnant am I? Determining gestational age is one of the first steps in prenatal care, and several methods exist to calculate it. The answer depends on what information you have available, such as the date of your last menstrual period, a known conception date, or an IVF transfer record.

Pregnancy weeks are most commonly calculated from the first day of the last menstrual period (LMP), and the standard estimated due date is 280 days, or 40 weeks, after that date, according to the American College of Obstetricians and Gynecologists. If you know the date of conception instead, the usual calculation is 266 days, or 38 weeks, from conception.

Each method comes with its own level of accuracy, and understanding how they work can help you interpret your results. This article explains the main calculation approaches, their reliability, and what to do when you are unsure of your dates.

How Many Weeks Pregnant Am I Based on My Last Period (LMP)?

Your Pregnancy Weeks at a Glance
Method	Input Needed	How It Works	Accuracy
Last Menstrual Period (LMP)	First day of last period	Count 280 days (40 weeks) from LMP	Best for regular cycles; variation +/- 1 week
Due Date	Estimated due date	Calculate backward from due date	Depends on due date accuracy
Conception Date	Date of ovulation or fertility window	Add 266 days (38 weeks)	If conception date is known precisely
IVF Transfer Date	Date of embryo transfer	Add 266 days minus embryo age (e.g., day 5 transfer = 261 days)	Very accurate with known transfer date
Ultrasound Dating	Gestational age from early scan	Based on fetal measurements	Most accurate in first trimester (±5 days)

LMP is the standard method because most women know their period date, but it assumes a 28-day cycle with ovulation on day 14. The NHS and What to Expect base their calculators on LMP.
IVF calculations require subtracting the embryo’s age from 266 days (38 weeks). Calculator.net includes IVF transfer dates in its tool.
Ultrasound dating is considered more accurate than LMP because it measures the baby directly. First-trimester ultrasound has an accuracy of ±5 to 7 days.
A due date is just an estimate; only about 5 percent of babies are born on their exact due date, a figure widely cited in medical literature.

Quick Facts: Pregnancy Length & Calculation
Fact
Full-term pregnancy: 39–40 weeks (37–42 weeks considered normal).
LMP method adds 280 days (40 weeks).
Conception method adds 266 days (38 weeks).
IVF: add 261 days for a Day 5 blastocyst transfer.
Due date is 40 weeks from LMP, not from conception.
Trimester 1: weeks 1–13, Trimester 2: weeks 14–27, Trimester 3: weeks 28–40+.

How Many Weeks Am I If I Know My Due Date?

If you already have an estimated due date from your healthcare provider, you can calculate your current pregnancy week by counting backward from that date. For example, if your due date is 40 weeks from your LMP and you are 10 weeks away from that date, you are approximately 30 weeks pregnant.

This approach depends entirely on the accuracy of the original due date estimate. If the due date was calculated from a reliable LMP or an early ultrasound, the backward calculation can be reasonably accurate. If the due date was estimated later in pregnancy or with limited information, the result may be less precise.

Using a Due Date Backward

To calculate current weeks from a due date, subtract the number of weeks remaining until that date from 40. If you have 8 weeks left, you are 32 weeks pregnant. This method works best when the due date itself was established with a reliable method such as first-trimester ultrasound.

How Do I Convert Pregnancy Weeks to Months?

A pregnancy month is roughly 4.3 weeks long. Month 1 covers weeks 1 through 4, month 2 covers weeks 5 through 8, month 3 covers weeks 9 through 13, and so on. Because months are not exactly 4 weeks, a pregnancy month calculator can help map weeks to months more precisely. Many online tools let you toggle between weeks and months to see where you stand.

How Many Weeks Pregnant Am I After IVF or Conception?

For those who conceived through IVF or who know their exact conception date, the standard LMP method may not apply. Conception occurs around the time of ovulation, which is typically about two weeks after the LMP. Therefore, pregnancy dating from conception uses a baseline of 266 days, or 38 weeks, instead of 280 days.

Conception Date Method

If you know the date of conception, add 266 days to that date. This gives you an estimated due date that aligns with fetal age rather than clinical gestation. Because conception date is often difficult to pinpoint without fertility tracking or assisted reproduction, this method is less commonly used as a primary tool.

IVF Transfer Date Method

For IVF pregnancies, the embryo transfer date provides a highly reliable starting point. According to calculator.net and the American Pregnancy Association, you add 263 days for a day-3 embryo transfer and 261 days for a day-5 blastocyst transfer. This approach offers the highest certainty among all dating methods, with an accuracy of about ±1 day, because the exact moment of fertilization and transfer is known.

IVF Dating in Practice

If you had a day-5 blastocyst transfer on June 1, add 261 days to find your estimated due date. To determine how many weeks pregnant you are, count the number of days since the transfer and divide by 7 to convert to weeks. This method is considered the most precise because the start point is documented medically.

How Accurate Are These Pregnancy Week Calculations?

Accuracy varies significantly by method and individual circumstances. LMP-based weeks are uncertain if cycles are irregular, meaning cycles shorter than 21 days or longer than 35 days. Ultrasound dating can shift the estimated due date and therefore the current week count, especially if the scan is performed after 20 weeks. Due date calculations have a margin of error of about ±2 weeks even with perfect data.

IVF transfer dates provide the highest certainty, at ±1 day. For everyone else, the week number should be treated as an estimate rather than an exact count. Healthcare providers typically rely on a combination of LMP and early ultrasound to assign the most accurate dates.

When Accuracy Decreases

If you have irregular cycles, LMP dating becomes less reliable. An adjusted approach using ultrasound is often used instead. One source from ultrasound.ie describes a formula: LMP + 280 days + (cycle length − 28 days). Another source notes that first pregnancies average 288 days and subsequent pregnancies average 283 days, though these figures are not standard clinical defaults.

What If I Do Not Know My Last Period?

If the LMP is unknown, an early ultrasound scan, ideally between 8 and 14 weeks, is the most accurate way to establish gestational age. The American College of Obstetricians and Gynecologists recommends first-trimester ultrasound for dating when the LMP is uncertain or when cycles are irregular. Some calculators also allow you to work from a due date or conception date if either is known.

How Does Ultrasound Dating Compare to LMP?

First-trimester ultrasound measures the embryo or fetus directly, providing an accuracy of about ±5 to 7 days. This makes it more reliable than LMP for many women. If an ultrasound date differs significantly from the LMP-based date, providers often adjust the due date to match the ultrasound findings, particularly when the scan is performed early.

What Happens from LMP to Birth? A Week-by-Week Timeline

Week 1–2: Conception occurs around the end of week 2.
Week 4: Missed period; a positive pregnancy test is possible.
Week 12: End of the first trimester; the NT scan is typically performed.
Week 20: Anatomy scan; this is the midpoint of pregnancy.
Week 28: Start of the third trimester.
Week 40: Estimated due date.
Week 42: Post-term; induction is often considered after this point.

How Certain Is the Number of Weeks You Are Pregnant?

Established Information	Information That Remains Unclear
LMP dating provides a standard 280-day baseline widely used in clinical practice.	LMP accuracy drops significantly with irregular cycles (under 21 days or over 35 days).
First-trimester ultrasound offers the highest accuracy (±5–7 days) when LMP is uncertain.	Ultrasound dating after 20 weeks becomes less precise and may shift the due date.
IVF transfer dates give the most precise starting point, with a margin of about ±1 day.	Even with perfect data, due date estimates carry a natural margin of error of about ±2 weeks.
Most doctors rely on LMP first because it is simple and widely understood.	The adjustment formula for cycle length (LMP + 280 + (cycle − 28)) is not universally adopted.

Why Do Different Pregnancy Dating Methods Give Different Results?

The apparent discrepancy arises because how many weeks pregnant you are can be measured from two different starting points: the LMP, which represents clinical gestation, or conception, which represents fetal age. Clinical gestation always shows about two weeks more than fetal age because it counts from the first day of the last period, which occurs roughly two weeks before ovulation and fertilization.

This difference explains why the LMP method gives 40 weeks while the conception method gives 38 weeks. Understanding this helps interpret calculator results and conversations with your healthcare provider. If your doctor says you are 12 weeks pregnant by LMP, your fetal age is approximately 10 weeks.

What Do Leading Health Authorities Say About Pregnancy Dating?

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

— NHS, NHS Due Date Calculator

“Most pregnancies last around 40 weeks (or 38 weeks from conception), so typically the best way to estimate your due date is to count 40 weeks, or 280 days, from the first day of your last period.”

— What to Expect, Due Date Calculator

“This calculator provides an estimated pregnancy schedule based on the due date, last period date, ultrasound date, conception date, or IVF transfer date.”

— Calculator.net, Pregnancy Calculator

How Can You Determine How Many Weeks Pregnant You Are?

The most reliable approach is to start with your LMP if your cycles are regular and you are certain of the date. If your cycles are irregular or you do not know your LMP, schedule an early ultrasound for accurate dating. For IVF pregnancies, use the embryo transfer date for the highest precision. Cross-check your results with a trusted Pregnancy Due Date Calculator that supports multiple methods, and discuss any discrepancies with your healthcare provider.

Frequently Asked Questions

What if I don’t know my last period date?

You can use an ultrasound dating scan, usually at 8 to 14 weeks, to determine gestational age. Some calculators also work from a due date or conception date if those are known.

How do I convert pregnancy weeks to months?

A pregnancy month is roughly 4.3 weeks. Month 1 = weeks 1–4, month 2 = weeks 5–8, month 3 = weeks 9–13, and so on. A pregnancy month calculator can help with exact mapping.

Why is my due date different on different calculators?

Different calculators may assume a different cycle length or use rounding. The standard is 280 days from LMP, but some tools adjust for conception or use 40 weeks exactly.

Can I be 40 weeks pregnant for more than a week?

Yes. Term includes weeks 37 to 42. Post-term is after 42 weeks, and inductions are often considered after 41 weeks.

How many weeks pregnant am I if I have irregular cycles?

LMP-based dating becomes less reliable. An early ultrasound is recommended. An adjustment formula exists: LMP + 280 days + (cycle length − 28 days).

Is ultrasound dating more accurate than LMP?

Yes, especially in the first trimester. Ultrasound measures the fetus directly and has an accuracy of about ±5 to 7 days, compared to LMP which assumes a 28-day cycle.

What is the difference between LMP weeks and conception weeks?

LMP weeks count from the first day of your last period (40 weeks total). Conception weeks count from fertilization (38 weeks total). The difference is about two weeks.

How do I calculate my weeks from an IVF transfer?

For a day-5 blastocyst transfer, add 261 days to the transfer date. For a day-3 embryo transfer, add 263 days. Then divide the total days since transfer by 7 to get weeks.

What if my ultrasound date and LMP date disagree?

Healthcare providers often adjust the due date to match the ultrasound if the scan was performed in the first trimester, as it is considered more reliable.

How accurate is the conception date method?

It is accurate only if you know the exact date of ovulation or conception. For most women, this is difficult to determine without fertility tracking or assisted reproduction.