Pregnancy Week Calculator

What the Pregnancy Week Calculator does (and what you need to enter)

On ProcalcAI, you enter: - Last Period Year - Last Period Month - Last Period Day

From that single date, the calculator estimates: - Your pregnancy age as weeks and days (for example, 10w 3d) - Your trimester - An estimated due date (LMP + 280 days) - Days left until the due date - Pregnancy progress as a percentage of a 280-day pregnancy - A fun “baby size” comparison tied to the week number

Important context: this is a date-based estimate, not a medical diagnosis. If you have irregular cycles, don’t remember your LMP, or conceived via assisted reproduction, a clinician may date the pregnancy differently (often by early ultrasound). The 280-day convention is widely used (40 weeks from LMP). (Silver: Mayo Clinic; Gold: NIH/MedlinePlus)

Step-by-step: how the calculator computes your pregnancy week

### 1) Create the LMP date Your inputs (year, month, day) are used to build a calendar date: - LMP = (Year, Month, Day)

If any field is missing, the internal logic uses defaults (Year 2025, Month 12, Day 1). In real use, you should always enter your actual LMP date to avoid nonsense results.

### 2) Find the number of days since LMP The calculator compares “today” to your LMP: - Difference in milliseconds = Today − LMP - Convert to days = floor(Difference / 86,400,000)

Then it prevents negative results (for example, if you accidentally enter a future LMP date): - Total days = max(0, calculated days)

So if your LMP was 50 days ago, total days = 50.

### 3) Convert total days into weeks and extra days Pregnancy age is expressed as full weeks plus leftover days: - Weeks pregnant = floor(total days / 7) - Extra days = total days mod 7

Example: total days 50 - weeks = floor(50/7) = 7 - days = 50 mod 7 = 1 Result: 7w 1d

### 4) Determine the trimester The calculator uses week cutoffs: - 1st Trimester: weeks < 13 - 2nd Trimester: weeks < 27 - 3rd Trimester: weeks ≥ 27

Note: Different organizations sometimes define trimester boundaries slightly differently (for example, some consider week 14 the start of the second trimester). ProcalcAI’s cutoffs are clear and consistent for calculation.

### 5) Estimate the due date (LMP + 280 days) The due date is calculated by adding 280 days to the LMP: - Due date = LMP + 280 days

This reflects the common “40 weeks from LMP” convention. (Silver: Mayo Clinic)

### 6) Calculate days left until due date The calculator computes: - daysLeft = ceil((dueDate − today) / 86,400,000) - then clamps to non-negative: max(0, daysLeft)

Using “ceil” means if you have 10.2 days left, it shows 11 days left (rounding up).

### 7) Calculate pregnancy progress percentage Progress is based on a 280-day pregnancy: - progress = round((totalDays / 280) × 100) - capped at 100%

So if totalDays = 70: - progress = round((70/280)×100) = round(25) = 25%

### 8) Assign a “baby size” by week number The calculator maps your week number to a list (poppy seed, sesame seed, lentil, … up to watermelon). It uses: - size index = min(weeks, last index in list)

So if you’re week 10, it picks the 11th item (since indexing starts at 0 internally). If you’re past the list range, it uses the final item.

Worked examples (using real math)

### Example 1: Early pregnancy (7w 1d) - LMP: 2026-01-01 - Today: 2026-02-20

1) Count days from Jan 1 to Feb 20: January has 31 days. From Jan 1 to Feb 20 is 50 days (Jan 1 → Feb 20). So totalDays = 50.

2) Convert to weeks and days: - weeks = floor(50/7) = 7 - days = 50 mod 7 = 1 Result: 7w 1d

3) Trimester: weeks 7 < 13 → 1st Trimester

4) Due date: LMP + 280 days → 2026-10-08 (since 2026 is not a leap year)

5) Progress: round((50/280)×100) = round(17.857) = 18%

### Example 2: Mid-pregnancy (20w 0d) - LMP: 2025-10-27 - Today: 2026-03-16

Compute total days: - Oct 27 → Oct 31 = 4 days - Nov = 30 - Dec = 31 - Jan = 31 - Feb 2026 = 28 - Mar 1 → Mar 16 = 15 days TotalDays = 4 + 30 + 31 + 31 + 28 + 15 = 139

Weeks and days: - weeks = floor(139/7) = 19 - days = 139 mod 7 = 6 Result: 19w 6d (very close to 20 weeks)

Trimester: 19 < 27 → 2nd Trimester

Due date: - 2025-10-27 + 280 days = 2026-08-03

Progress: - round((139/280)×100) = round(49.64) = 50%

### Example 3: Late pregnancy (36w 3d) - LMP: 2025-07-01 - Today: 2026-03-13

Total days from 2025-07-01 to 2026-03-13: - Jul 2025: 31 - Aug: 31 (62) - Sep: 30 (92) - Oct: 31 (123) - Nov: 30 (153) - Dec: 31 (184) - Jan 2026: 31 (215) - Feb 2026: 28 (243) - Mar 1–13: 12 (255)

totalDays = 255

Weeks and days: - weeks = floor(255/7) = 36 - days = 255 mod 7 = 3 Result: 36w 3d

Trimester: 36 ≥ 27 → 3rd Trimester

Due date: - 2025-07-01 + 280 days = 2026-04-07

Days left (from Mar 13 to Apr 7): 25 days

Progress: - round((255/280)×100) = round(91.07) = 91%

Pro Tips for getting the most accurate result

Common mistakes (and how to avoid them)

Sources: Mayo Clinic (pregnancy due date and gestational age overview); NIH/MedlinePlus (pregnancy weeks and prenatal care timing).

Authoritative Sources

This calculator uses formulas and reference data drawn from the following sources:

- CDC — Physical Activity - NIH — National Institute of Diabetes - NIH — National Heart, Lung, and Blood Institute

This pregnancy week calculator uses standard health formulas to compute results. Enter your values and the formula is applied automatically — all math is handled for you. The calculation follows industry-standard methodology.

• https://www.nih.gov
• https://www.who.int
• https://www.cdc.gov