Calculator Suite
Pregnancy Weeks Calculator
Calculate gestational age, due date, and track fetal development milestones
Common Scenarios
Key Milestones
Medical Disclaimer
This calculator provides estimates based on standard pregnancy calculations. Always consult with your healthcare provider for personalized medical advice, accurate dating via ultrasound, and pregnancy monitoring.
Naegele's Rule (Standard Due Date)
= Estimated Date of Delivery
= First day of Last Menstrual Period
Example: LMP Jan 1 → Subtract 3 months (Oct 1) → Add 7 days (Oct 8).
Gestational Age (Medical Dating)
Doctors count pregnancy from the start of your period, about 2 weeks before you actually accidentally conceived.
Conception Adjustment
Standard pregnancy is 280 days from LMP, but only ~266 days from conception.
TL;DR: Pregnancy is typically 40 weeks long. We count from the first day of your last period (LMP), not the day you conceived. This standard method (Naegele's Rule) helps doctors track baby's growth milestones accurately.
Real-World Example: Jane's Timeline
Jane discovers she is pregnant. Here is how her dates break down:
- Last Period Started: January 1st
- Likely Ovulation/Conception: ~January 14th (Day 14)
- Positive Test: ~January 28th (Week 4)
The Calculation:
- Math: Jan 1 - 3 months = Oct 1. + 7 days = Oct 8.
- Due Date: October 8th
- Reality Check: Only ~5% of babies are born on the exact due date. Jane should be ready anytime from late September to mid-October.
3 Key Checks: Is this Accurate?
- Cycle Length Matters: The standard calculation assumes a 28-day cycle. If yours is long (e.g., 35 days), you likely ovulated later, so your "real" due date might be ~7 days after the calculated date.
- LMP vs. Ultrasound: If you don't remember your LMP exact date, or have irregular periods, the dating ultrasound (usually at 8-12 weeks) is the gold standard. Doctors will adjust your due date if the baby measures big/small discrepancies more than 7 days.
- "The 2-Week Gap": Remember, you aren't technically pregnant during "Week 1" and "Week 2." Those are the weeks of your period and ovulation. It's weird, but it's how medicine works!
Assumptions & Limitations
- 28-day menstrual cycle
- Ovulation occurs on day 14
- Standard 40-week gestation
- Delayed ovulation (stress/illness)
- Cycle irregularity
- Early induction or preterm labor risks
TED-Ed - The Surprising Effects of Pregnancy
Source: TED-Ed
The Biology of Prenatal Development
Source: EHD
Frequently Asked Questions
How accurate is this due date?
It is an estimate. Studies show only ~4-5% of babies are born on their exact EDD. Most (80%) are born within the window of 38 to 42 weeks. Treat it as a "due month" rather than a specific day.
Can I calculate from conception date?
Yes. If you know exactly when you conceived (e.g., IVF or tracking ovulation), use the "Conception Date" method in the calculator toggle. It is often more accurate than LMP for women with irregular cycles.
When will I hear the heartbeat?
A vaginal ultrasound can often detect a heartbeat as early as 6 weeks. A handheld doppler (at checkups usually) picks it up around 10-12 weeks.
Why am I 4 weeks pregnant if I only missed my period yesterday?
Because the medical calendar starts from your last period. The first 2 weeks you weren't pregnant, but they count toward the 40-week total. So by the time you miss a period, you are already considered "4 weeks" along.