Calculation of Pregnancy – How to Calculate Pregnancy Weeks
Calculation of Pregnancy: How to Calculate Pregnancy Weeks and Due Date
Obstetric gestational age is counted from the first day of the last menstrual period (LMP), not from conception. A typical pregnancy lasts 40 weeks (280 days), about 9 calendar months plus one week, or ten four-week months, rather than nine months.
Clinical Formulas for Calculation of Pregnancy
Naegele’s Rule
- Take the first day of the LMP.
- Add 1 year.
- Subtract 3 months.
- Add 7 days.
Example:
LMP March 10, 2026. Add 1 year (March 10, 2027), subtract 3 months (December 10, 2026), add 7 days: December 17, 2026.
Because months differ in length, Naegele’s rule can differ from an exact 280-day count by 1–3 days. Counting 280 days from March 10, 2026 gives December 15, 2026.
Adjusting for Non-28-Day Cycles
Naegele’s rule assumes ovulation on day 14. Adjust by the difference from 28 days:
- 35-day cycle (LMP March 10): December 17 + 7 = December 24, 2026
- 24-day cycle: December 17 − 4 = December 13, 2026
Parikh’s Formula
Parikh’s formula is a variation taught in some obstetric curricula: add 9 months to the LMP, subtract 21 days, and add the woman’s cycle length in days. For a 28-day cycle, this gives LMP + 9 months + 7 days, the same as Naegele’s rule. For a 35-day cycle, it gives LMP + 9 months + 14 days, matching the cycle-adjusted Naegele’s calculation above. It requires a known, fairly regular cycle length. For truly irregular cycles, ultrasound dating is preferred. See Ultrasound Due Date Accuracy.
Weeks, Months, and Trimesters
40 weeks is 280 days, or 9.2 calendar months. Because 4 weeks is shorter than most calendar months, grouping by month is approximate. The calendar-month grouping below is the one most commonly used by patients.
| Trimester | Month | Week Range | Key Developmental Milestones |
|---|---|---|---|
|
First Trimester
|
Month 1 | Weeks 1–4 | Ovulation and fertilization (week 3); blastocyst implantation; positive urine pregnancy test detectable around week 4. |
| Month 2 | Weeks 5–8 | Active organogenesis; early embryonic cardiac activity detectable on transvaginal ultrasound at approximately 6 weeks. | |
| Month 3 | Weeks 9–13 | Fetal development period begins at week 10; optional nuchal translucency (NT) screening; first trimester completes at 13w 6d. | |
|
Second Trimester
|
Month 4 | Weeks 14–17 | Rapid skeletal and muscular growth; initial subtle fetal movements (quickening) may begin in multiparous women. |
| Month 5 | Weeks 18–21 | Comprehensive mid-pregnancy fetal anatomy survey ultrasound (routinely scheduled between 18 and 22 weeks). | |
| Month 6 | Weeks 22–26 | Clinical fetal viability milestone approached (~24 weeks); universal gestational diabetes glucose screening (24–28 weeks). | |
|
Third Trimester
|
Month 7 | Weeks 27–30 | Third trimester begins officially at 28w 0d; progressive alveolar lung maturation and surfactant production. |
| Month 8 | Weeks 31–35 | Accelerated fetal subcutaneous fat deposition; spontaneous cephalic (head-down) positioning preparation. | |
| Month 9 | Weeks 36–40 | Early term reaches at 37w 0d–38w 6d; full-term delivery benchmark established at 39w 0d–40w 6d. |
40 weeks is ten four-week “lunar” months. In calendar terms, nine months from LMP is about 39 weeks, so a due date at 40 weeks falls about a week into month 10.
Use the Pregnancy Trimester Timeline to see your current week and trimester, and Conception Date vs. LMP to convert between the two dating systems.
Am I in Week 5 or Week 6?
Pregnancy age is reported as completed weeks plus days. “5 weeks 3 days” (5w3d) means 5 full weeks and 3 extra days have passed, so you are in your 6th week of pregnancy.
| LMP-Based Day Count | Clinical Notation (GA) | Week You Are In |
|---|---|---|
| 35 days | 5w 0d | 6th week |
| 38 days | 5w 3d | 6th week |
| 41 days | 5w 6d | 6th week |
| 42 days | 6w 0d | 7th week |
To calculate: Days since LMP ÷ 7 = completed weeks (whole number); the remainder is days. Example: 94 days ÷ 7 = 13 weeks, remainder 3, which is 13w3d.
Frequently Asked Questions for Calculation of Pregnancy
Why are you considered 2 weeks pregnant before you conceive?
The LMP is the only reliably known date for most women. Ovulation and conception happen about 14 days after it. Counting from LMP is a convention that standardizes dating worldwide. The first two weeks include menstruation and follicular development, before fertilization.
How many weeks is 9 months in Calculation of Pregnancy weeks?
About 39 weeks from LMP (9 calendar months), or about 38 weeks from conception. Many people refer to the whole pregnancy as “nine months,” but 40 weeks is closer to 9 months and 1 week, or 10 four-week months.
How do doctors calculate your due date if you don’t know your LMP?
They use ultrasound. A first-trimester ultrasound CRL is the most accurate method, within about ±5–7 days. Later, estimates are less precise and use BPD, HC, AC, and FL. Fundal height, the date of a positive test, and the date of first fetal movement (“quickening”) are supportive but not precise. In IVF, the embryo transfer date determines the due date.
Does a 40-week due date mean the baby will arrive on that exact day?
No. Only about 4% of babies are born on their exact EDD. Most births occur within about 2 weeks of it. “Full term” spans 39w0d to 40w6d, and delivery between 37 and 42 weeks is considered within the normal range.
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