How to Calculate Pregnancy Weeks — Due Date & Trimester Guide

"How many weeks am I?" is one of the most common questions during pregnancy. The answer seems straightforward but has a quirk: pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. This means you're technically "2 weeks pregnant" before you've even conceived.

Here's how to calculate your pregnancy weeks accurately. Use the CalcHub pregnancy calculator for instant results.

The Basic Calculation

Pregnancy Weeks = (Today's Date - LMP Date) / 7

Example:

• Last menstrual period started: January 15, 2026
• Today's date: March 28, 2026
• Days since LMP: 72 days
• Pregnancy weeks: 72 / 7 = 10 weeks and 2 days

You'd be described as "10+2 weeks" or "10 weeks 2 days."

Why Counting Starts from LMP

Ovulation typically occurs around day 14 of a 28-day cycle. Conception happens around this time. But since most women don't know their exact ovulation date, doctors use the LMP — a date most women can identify with certainty.

This means:

• Week 1-2: You're not actually pregnant yet — this is before ovulation


• Week 3: Fertilization occurs


• Week 4: Implantation — around when you'd miss your period


• Week 40: Your due date

Trimester Breakdown

Trimester	Weeks	Key Developments
First	1-12	Organ formation, heartbeat begins (week 6), major development
Second	13-27	Growth acceleration, movements felt (week 16-20), anatomy scan
Third	28-40	Weight gain, lung maturation, preparation for birth

Calculating from Conception Date

If you know your conception date (from IVF, ovulation tracking, or a known date):

Pregnancy Weeks = ((Today's Date - Conception Date) / 7) + 2

The "+2" adjusts for the 2-week difference between LMP dating and conception dating.

Example:

• Conception date: February 1, 2026
• Today: March 28, 2026
• Days since conception: 55
• Weeks since conception: 7 weeks 6 days
• Pregnancy weeks (LMP equivalent): 9 weeks 6 days

Calculating Your Due Date

Due Date = LMP + 280 days (40 weeks)

Or using Naegele's Rule:

1. Take the first day of your LMP


2. Add 7 days


3. Subtract 3 months


4. Add 1 year

Example:

• LMP: January 15, 2026


• Add 7 days: January 22


• Subtract 3 months: October 22


• Due date: October 22, 2026

Adjusting for Irregular Cycles

The standard calculation assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, adjust:

Adjusted LMP = Actual LMP + (Cycle Length - 28)

For a 35-day cycle:

• Actual LMP: January 15


• Adjusted LMP: January 15 + 7 = January 22


• This shifts your due date 7 days later

Your doctor will confirm or adjust the date with a first-trimester ultrasound, which is accurate to within 5-7 days.

Important Milestones by Week

Week	Milestone
6	Heartbeat detectable on ultrasound
8	All major organs forming
12	End of first trimester; risk of miscarriage drops significantly
16-20	First movements felt (quickening)
20	Anatomy scan / mid-pregnancy ultrasound
24	Viability milestone
28	Third trimester begins
37	Considered "early term" — baby is ready
39-40	Full term

Practical Tips

1. Don't panic about the exact date. Only about 5% of babies arrive on their calculated due date. Most full-term births occur between 37 and 42 weeks. Think of the due date as an estimate, not a deadline. 2. Use your doctor's dating, not just the calculator. If your first-trimester ultrasound gives a different date than the LMP calculation, your doctor will use the ultrasound date — it's more accurate, especially with irregular cycles. 3. "Weeks pregnant" vs "the Xth week." Being "12 weeks pregnant" means you've completed 12 weeks and are in your 13th week. This causes confusion — always clarify whether you mean completed weeks.

Why does my doctor's week count differ from my app?

This usually happens because of different starting points. Some apps use LMP, some use estimated conception date, and some adjust for cycle length. Your doctor's calculation, especially if based on ultrasound, takes priority.

Can I calculate pregnancy weeks if I don't remember my LMP?

If you have no idea when your last period was, an early ultrasound (ideally before 12 weeks) is the most reliable way to date your pregnancy. The doctor measures the embryo/fetus size and estimates gestational age.

Is there a difference between gestational age and fetal age?

Yes. Gestational age (what doctors and calculators use) counts from LMP and is about 2 weeks longer than fetal age (which counts from actual conception). When someone says "8 weeks pregnant," they mean 8 weeks gestational age, but the embryo has been developing for about 6 weeks.

Related Calculators

• Pregnancy Due Date Calculator — calculate your expected delivery date
• Age Calculator — calculate exact age in years, months, days
• Date Difference Calculator — count days between dates
• BMI Calculator — track healthy weight during pregnancy