How to Calculate Pregnancy Weeks: Methods, Due Dates & Trimesters

Figuring out how far along you are is one of the first things every pregnant person wants to know. Pregnancy is counted in weeks, not months, and the starting point often surprises people — it begins before conception actually occurs. Whether you know your last menstrual period (LMP), your conception date, or you have an ultrasound result, this guide explains exactly how to calculate your pregnancy weeks, determine your due date, and understand what trimester you’re in.

How Pregnancy Weeks Are Counted

Pregnancy is measured from the first day of your last menstrual period (LMP), not from the day of conception. This is the standard used by obstetricians worldwide and is the basis for all gestational age calculations.

The reason for this: ovulation and fertilization typically occur around day 14 of a 28-day cycle, but the exact date of conception is rarely known with certainty. The LMP date is almost always known, making it a reliable and consistent starting point.

A full-term pregnancy lasts 40 weeks from the LMP (or approximately 38 weeks from conception).

Method 1: Calculate from Last Menstrual Period (LMP)

This is the most common and medically standard method.

Gestational Age (in weeks) = (Today’s Date − First Day of LMP) ÷ 7

Step-by-Step Example

• Last menstrual period started: March 1, 2026
• Today’s date: May 1, 2026

Step 1 — Count the total days between LMP and today: March 1 to May 1 = 31 days (March) + 30 days (April) = 61 days

Step 2 — Divide by 7 to convert to weeks: 61 ÷ 7 = 8 weeks and 5 days (often written as 8w5d)

You are currently 8 weeks and 5 days pregnant.

Use our free Pregnancy Calculator to get your exact gestational age and due date instantly.

Method 2: Calculate from Conception Date

If you know your conception date (from tracking ovulation or fertility treatment), you can calculate gestational age by adding approximately 14 days to the conception date to get the equivalent LMP-based start, then counting from there.

Gestational Age = Conception Date + 14 days, then count forward

Alternatively: Gestational Age (weeks) = (Today’s Date − Conception Date) ÷ 7 + 2

The +2 accounts for the two weeks between LMP and typical ovulation.

Example

• Conception date: March 15, 2026
• Today’s date: May 1, 2026

Days since conception: March 15 to May 1 = 47 days 47 ÷ 7 = 6.7 weeks since conception Add 2 weeks: 6.7 + 2 = approximately 8 weeks and 5 days pregnant

This aligns with Method 1, as expected.

Method 3: Using Ultrasound Measurements

An ultrasound in the first trimester (before 14 weeks) is the most accurate way to determine gestational age. The sonographer measures the crown-rump length (CRL) of the embryo and compares it to established growth charts.

First-trimester ultrasounds are accurate to within 3–5 days. After the first trimester, accuracy decreases because fetal growth varies more between individuals.

If your ultrasound result differs from your LMP-based calculation by more than a week, your doctor may adjust your official due date to match the ultrasound measurement.

How to Calculate Your Due Date (Naegele’s Rule)

The standard formula for calculating a due date is Naegele’s Rule:

Due Date = First Day of LMP + 280 days (40 weeks)

A simpler way to apply it manually:

Due Date = LMP + 9 months + 7 days

Example

• LMP: March 1, 2026
• Add 9 months: December 1, 2026
• Add 7 days: December 8, 2026

Your estimated due date (EDD) is December 8, 2026.

Naegele’s Rule assumes a standard 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, your doctor may adjust the due date accordingly.

The Three Trimesters Explained

Pregnancy is divided into three trimesters, each roughly 13 weeks long:

Trimester	Weeks	Key Developments
First Trimester	Weeks 1 – 13	Implantation, organ formation, heartbeat detectable (~week 6)
Second Trimester	Weeks 14 – 27	Rapid growth, fetal movement felt, anatomy scan (~week 20)
Third Trimester	Weeks 28 – 40+	Weight gain, lung development, baby positions for birth

First Trimester (Weeks 1–13)

The first trimester begins with your LMP — before you are technically pregnant. Fertilization occurs around week 2. By week 4–5, a pregnancy test will typically turn positive. Morning sickness, fatigue, and breast tenderness are common during this phase.

Key milestones:

• Week 6: Heartbeat usually visible on ultrasound
• Week 8: All major organs have begun forming
• Week 10: Embryo is now called a fetus
• Week 12–13: First trimester screening (nuchal translucency scan)

Second Trimester (Weeks 14–27)

Most people find the second trimester the most comfortable. Morning sickness typically eases, energy returns, and the pregnancy becomes visible. This is when most parents find out the baby’s sex.

Key milestones:

• Week 16–20: Fetal movements (quickening) felt for the first time
• Week 20: Anatomy ultrasound scan
• Week 22: Fetus is considered viable if born prematurely
• Week 24–28: Glucose screening test for gestational diabetes

Third Trimester (Weeks 28–40+)

The third trimester is marked by significant weight gain for both mother and baby. The baby’s lungs mature, and the body prepares for labor.

Key milestones:

• Week 28: Third trimester begins; kick counting recommended
• Week 32–34: Baby likely moves into head-down position
• Week 36: Considered “early term”
• Week 37: Full term begins
• Week 40: Estimated due date
• Week 42: Post-term; induction typically recommended

Pregnancy Week-by-Week Size Reference

Fetal size is often compared to fruits and vegetables as a fun reference point:

Week	Approximate Size	Length (approx.)
6	Sweet pea	0.6 cm
8	Kidney bean	1.6 cm
10	Strawberry	3.1 cm
12	Lime	5.4 cm
16	Avocado	11.6 cm
20	Banana	25.6 cm
24	Ear of corn	30 cm
28	Eggplant	37.6 cm
32	Squash	42.4 cm
36	Romaine lettuce	47.4 cm
40	Watermelon	~51 cm

How Cycle Length Affects Your Due Date

Naegele’s Rule assumes a 28-day cycle. If your cycle is different, your actual ovulation date shifts, which affects when conception occurred and therefore the true gestational age.

Cycle Length	Ovulation Day	Adjustment to Due Date
21 days	Day 7	Subtract 7 days from Naegele’s date
28 days	Day 14	No adjustment (standard)
35 days	Day 21	Add 7 days to Naegele’s date
40 days	Day 26	Add 12 days to Naegele’s date

If you have irregular cycles or used fertility treatments, tell your doctor — they will use your known ovulation or transfer date to set the due date more precisely.

Converting Pregnancy Weeks to Months

Pregnancy is clinically measured in weeks, but many people think in months. The conversion is not as clean as dividing by 4, because months have different lengths:

Months Pregnant	Approximate Weeks
1 month	Weeks 1 – 4
2 months	Weeks 5 – 8
3 months	Weeks 9 – 13
4 months	Weeks 14 – 17
5 months	Weeks 18 – 22
6 months	Weeks 23 – 27
7 months	Weeks 28 – 31
8 months	Weeks 32 – 35
9 months	Weeks 36 – 40

“9 months pregnant” is actually anywhere from weeks 36 to 40+, which is why healthcare providers stick to weeks for precision.

What If I Don’t Know My LMP?

If your period was irregular or you don’t remember your LMP, there are other ways to establish gestational age:

• First trimester ultrasound — Most accurate option before 14 weeks; crown-rump length pinpoints age to within a few days
• hCG blood test levels — Can give a rough gestational range but not a precise week count
• Known ovulation date — From ovulation predictor kits, basal body temperature tracking, or fertility monitoring
• IVF transfer date — For IVF pregnancies, gestational age is calculated from the egg retrieval date plus two weeks (for a day-2 or day-3 transfer) or directly from the transfer date using adjusted formulas

Common Questions About Pregnancy Week Calculation

How do I know what week of pregnancy I’m in?

Count the days from the first day of your last menstrual period to today, then divide by 7. The whole number is your completed weeks; the remainder is the extra days. Our Pregnancy Calculator does this automatically.

Am I 4 weeks pregnant or 6 weeks pregnant when I miss my period?

When you miss your first period (typically 4 weeks after your LMP), you are considered 4 weeks pregnant by gestational age. If your LMP was 6 weeks ago, you are 6 weeks pregnant — even though fertilization only happened about 4 weeks ago.

Is 40 weeks exactly 9 months?

Not quite. 40 weeks is approximately 9 months and 1 week. This is why pregnancies are described in weeks rather than months to avoid confusion.

Can my due date change?

Yes. Your due date may be adjusted after your first ultrasound if the measured size of the embryo doesn’t match your LMP-based date. The ultrasound date takes precedence when there is a significant discrepancy (more than 5–7 days in the first trimester).

What does “gestational age” mean?

Gestational age is the age of the pregnancy measured from the LMP. It differs from “fetal age” (which is counted from conception) by about two weeks. All clinical pregnancy milestones — viability, trimester boundaries, due date — use gestational age.

What is a full-term pregnancy?

A full-term pregnancy is 37 to 40 weeks and 6 days. Births before 37 weeks are preterm; births from 41 weeks onward are late-term or post-term.

My ultrasound and LMP calculations disagree — which is correct?

In the first trimester, the ultrasound is more accurate. Your doctor will typically update your due date to match the ultrasound if they differ by more than 5 days before 9 weeks, or more than 7 days between 9 and 14 weeks.

The Bottom Line

Calculating pregnancy weeks starts with the first day of your last menstrual period. Count the days from that date to today and divide by 7 — that is your gestational age. Add 280 days (or 9 months and 7 days) to your LMP to get your estimated due date using Naegele’s Rule.

For irregular cycles or unknown LMP dates, a first-trimester ultrasound is the gold standard for dating a pregnancy. All trimester milestones, screening tests, and delivery planning are built around your gestational age in weeks.

Use our free Pregnancy Calculator to calculate your exact week, trimester, and estimated due date based on your LMP or conception date.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult your obstetrician or midwife for clinical guidance on your pregnancy.