How to Calculate Pregnancy Weeks
Quick Answer
To find how many weeks pregnant you are: count the days from the first day of your last menstrual period (LMP) to today, then divide by 7.
To find your estimated due date (EDD): add 280 days (40 weeks) to the first day of your last period.
Example: LMP = 1 March 2025. Due date = 1 March + 280 days = 5 December 2025.
Full-term pregnancy = 40 weeks from LMP = approximately 38 weeks from conception.
Knowing your pregnancy week is one of the first things healthcare providers assess at your initial antenatal appointment. This guide explains the calculation, the trimester timeline and what healthcare systems in different countries expect at each stage. Use the free Pregnancy Calculator and Due Date Calculator on CalConvs for instant results.
How Pregnancy Weeks Are Counted
Pregnancy is counted from the first day of your last menstrual period, not from the date of conception. This is because the exact date of conception is rarely known with certainty, while the LMP date is usually known and verifiable.
Because ovulation and conception typically occur around day 14 of a 28-day cycle, being 4 weeks pregnant means conception happened approximately 2 weeks ago. This is the universal standard used by all healthcare systems worldwide.
The Three Trimesters: What Happens When
| Trimester | Weeks and Months | Key Events |
|---|---|---|
| First trimester | Weeks 1 to 12 (0 to 3 months) | All major organs begin forming. Morning sickness common. Miscarriage risk is highest. Nuchal translucency scan typically at weeks 11 to 13. |
| Second trimester | Weeks 13 to 27 (3 to 6 months) | Growth accelerates. Movement felt from about week 18 to 20. Anomaly scan (anatomy scan) typically at week 20. |
| Third trimester | Weeks 28 to 40+ (6 to 9 months) | Rapid weight gain. Lungs mature. Baby moves into birth position. Pre-term is before 37 weeks; full-term is 37 to 42 weeks. |
Due Date Calculation: Naegele's Rule
Estimated Due Date = LMP + 280 days (or equivalently: LMP + 9 months + 7 days)
- Example 1: LMP = 15 January 2025. Due date = 22 October 2025.
- Example 2: LMP = 1 June 2025. Due date = 7 March 2026.
For cycles longer or shorter than 28 days, the due date shifts. A 35-day cycle shifts the due date 7 days later. A 21-day cycle shifts it 7 days earlier.
Antenatal Care by Country
- United Kingdom (NHS): Booking appointment at 8 to 10 weeks. Nuchal scan at 11 to 13 weeks. Anomaly scan at 18 to 21 weeks. Midwife-led care for uncomplicated pregnancies.
- United States: First prenatal visit at weeks 8 to 10. Ultrasound dating scan at weeks 8 to 12. Anatomy scan at week 20. More frequent OB visits in third trimester.
- Australia: Medicare-funded antenatal care. Dating scan at weeks 11 to 13. Morphology scan at weeks 18 to 20. Care through GP, midwife or obstetrician depending on risk level.
- India: Government guidelines recommend first visit in first trimester, then monthly visits. Ultrasound at first trimester and 18 to 20 weeks.
- Pakistan: Public health antenatal clinics provide free care. Key visits at confirmation, 12 weeks, 20 weeks, 28 weeks, 32 weeks and 36 weeks.
Healthy Pregnancy Weight Gain by BMI
| Pre-Pregnancy BMI | Recommended Total Gain |
|---|---|
| Underweight (BMI below 18.5) | 12.7 to 18.1 kg (28 to 40 lbs) |
| Normal weight (BMI 18.5 to 24.9) | 11.3 to 15.9 kg (25 to 35 lbs) |
| Overweight (BMI 25 to 29.9) | 6.8 to 11.3 kg (15 to 25 lbs) |
| Obese (BMI 30 or above) | 5 to 9 kg (11 to 20 lbs) |
Frequently Asked Questions
How do I know how many weeks pregnant I am?
Enter the first day of your last menstrual period into the Pregnancy Calculator on CalConvs. It will tell you exactly how many weeks pregnant you are today, which trimester you are in and your estimated due date. If you do not know your LMP, an ultrasound dating scan in the first trimester is the most accurate method.
What is considered full term?
A pregnancy is considered full term at 37 weeks of gestation or more. Deliveries between 37 and 42 weeks are considered within the normal range. Early term is 37 to 39 weeks, full term is 39 to 41 weeks, and late term is 41 to 42 weeks.
How accurate is the due date calculation?
The LMP-based due date is accurate to within about one to two weeks for women with regular 28-day cycles. Only about 5% of babies are born on their exact estimated due date. A first-trimester ultrasound (8 to 12 weeks) is more accurate and is used to adjust the due date when the ultrasound measurement differs significantly from the LMP date.
Related Tools
- Pregnancy Calculator: how many weeks pregnant and trimester tracking
- Due Date Calculator: find your estimated delivery date
- BMI Calculator: pre-pregnancy BMI for weight gain guidance
- All Health and Fitness Tools: 28 tools on CalConvs