Pregnancy Week Calculator — How Many Weeks Am I?
Find out exactly how many weeks pregnant you are today. Enter the first day of your last menstrual period (LMP) and this calculator returns your current gestational week, days, trimester, and upcoming milestones. Uses Naegele's rule (standard ACOG methodology).
How many weeks pregnant you are equals the completed weeks since the first day of your last menstrual period (LMP). Pregnancy is dated from LMP, not from conception, so week 1 begins about two weeks before you ovulate. A full-term pregnancy spans 40 weeks (280 days) under ACOG convention. A first-trimester ultrasound refines this count when cycles are irregular.
When to use this calculator
- You want to know exactly how many weeks pregnant you are today.
- You're tracking weekly baby development milestones.
- You need to plan prenatal appointments by week.
- You're preparing for the 20-week anatomy scan.
- Tracking when you enter each trimester.
Example: Sarah's LMP was January 1, 2026
- LMP: January 1, 2026.
- Today: April 21, 2026.
- Days since LMP: 110 days.
- Weeks: 15 weeks, 5 days.
- Trimester: 2nd trimester.
- EDD: October 8, 2026 (280 days from LMP).
How it works
1 min read¿Qué es la calculadora de semanas de embarazo?
Es una herramienta que determina tu semana gestacional actual restando la fecha de tu última menstruación (LMP) a hoy. Usa la regla de Naegele, el método estándar de ACOG: (hoy - LMP) ÷ 7 = semana actual. El embarazo promedio dura 40 semanas (280 días) desde el primer día del ciclo menstrual.
How to count pregnancy weeks
Pregnancy is counted in weeks, starting from the first day of your last menstrual period (LMP) — not from conception. This is the standard used by doctors worldwide.
Formula: Current week = floor((today - LMP) / 7)
So if your LMP was 110 days ago, you're in week 15 (110/7 = 15.7).
Trimester breakdown
| Trimester | Weeks | Key events |
|---|---|---|
| 1st | 1-13 | Organ formation, nausea, first heartbeat detected (week 6-7) |
| 2nd | 14-27 | Anatomy scan (week 20), gender reveal, first movements |
| 3rd | 28-40 | Growth, preparation for birth, baby positioning |
Key milestones by week
Why is it counted from LMP and not conception?
Because the exact day of conception is usually unknown, but the first day of LMP is a reliable marker. Conception actually occurs around week 2 (ovulation), but obstetricians count from week 1 = first day of LMP. This means at week 4, you're only 2 weeks pregnant biologically.
Is my due date exact?
No. Only 5% of babies are born on their exact due date. Normal range is 37-42 weeks. Your obstetrician may adjust the EDD based on early ultrasounds, which are more accurate in the first trimester.
Sources
Frequently asked questions
How many weeks am I pregnant if my last period was 8 weeks ago?
You are 8 weeks pregnant. Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from conception.
How accurate is this pregnancy calculator?
Very accurate if you know your LMP date. It uses Naegele's rule (LMP + 280 days), the gold standard in obstetrics. However, early ultrasounds (weeks 6-12) provide the most accurate dating and may adjust the EDD by a few days.
What if I have irregular periods?
LMP-based calculations are less accurate with irregular cycles. Ask your doctor for a dating ultrasound in the first trimester — it's the most reliable method for irregular cycles.
When does the second trimester start?
The second trimester starts at week 14 and ends at week 27. The first trimester covers weeks 1-13; the third covers weeks 28-40+.
When is the anatomy scan?
The detailed anatomy scan (Level II ultrasound) is typically done between week 18 and 22, usually at week 20. This is when you can also find out the baby's sex.
Can a pregnancy be longer than 40 weeks?
Yes. Normal pregnancy is considered 37-42 weeks. Past 40 weeks is "late-term", past 42 weeks is "post-term" and doctors usually recommend induction to prevent complications.
What is Naegele's rule?
Franz Karl Naegele (German obstetrician) developed the rule in 1812: EDD = LMP + 280 days (or LMP − 3 months + 7 days + 1 year). It's the standard formula used in pregnancy calculators.