Pregnancy Week Calculator by Due Date
Pregnancy week calculator finds how many weeks and days pregnant you are from your due date. Enter your due date to instantly see your current gestational week, trimester, estimated conception date, days remaining, and pregnancy percentage complete. A due date of July 15 entered on April 16 means you are 26 weeks and 2 days pregnant in the third trimester with 90 days remaining.
Enter Values
The year of your estimated due date
Month (1-12) of your due date
Day of the month of your due date
Result
Enter values above and click Calculate to see your result.
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Formula
How it works: A full-term pregnancy is 280 days (40 weeks) from the first day of the last menstrual period. By subtracting the days remaining until the due date from 280, the calculator determines how many gestational days have passed, then converts to weeks and days. The trimester is determined from the week count: weeks 1-12 are the first trimester, weeks 13-27 are the second trimester, and weeks 28-40 are the third trimester.
Worked Example
How to Calculate Pregnancy Weeks from Your Due Date
Pregnancy is measured in gestational weeks starting from the first day of your last menstrual period (LMP), totaling 40 weeks (280 days) for a full-term pregnancy. If you know your due date but not your LMP, this calculator works backward: it subtracts the days remaining until your due date from 280 to determine how many gestational days have elapsed, then converts that number into weeks and days. This is especially useful for women who were given a due date by their doctor from an ultrasound dating scan and do not remember their exact LMP date. The calculator also determines your current trimester, estimates your conception date (approximately 266 days before the due date), and shows your pregnancy progress as a percentage.
- Full-term pregnancy is 280 days (40 weeks) from LMP. Your due date is the endpoint of this 280-day countdown.
- First trimester covers weeks 1-12 (organ formation), second trimester weeks 13-27 (growth and movement), third trimester weeks 28-40 (weight gain and preparation for birth).
- Conception typically occurs around day 14 of a 28-day cycle, which is approximately 266 days before the due date.
- Only about 5% of babies are born on their exact due date. Most healthy births occur between weeks 37 and 42.
- Your doctor may adjust your due date based on ultrasound measurements, especially if performed in the first trimester when dating is most accurate.
Use this calculator alongside the Trimester Calculator for detailed milestone tracking and the Pregnancy Weight Gain Calculator to ensure you are gaining weight at a healthy rate for your stage of pregnancy.
You can also calculate changes using our Pregnancy Due Date Calculator, Gestational Age Calculator, Trimester Calculator, Conception Date Calculator, Pregnancy Month Calculator or Pregnancy Weight Gain Calculator.
Pregnancy Milestones by Week
Key developmental milestones throughout the 40 weeks of pregnancy.
| Week | Trimester | Milestone | Baby Size |
|---|---|---|---|
| 6 | 1st | Heartbeat detectable on ultrasound | Lentil (3 mm) |
| 12 | 1st | End of first trimester, organs formed | Lime (5 cm) |
| 16 | 2nd | Gender may be visible on ultrasound | Avocado (12 cm) |
| 20 | 2nd | Anatomy scan, halfway point | Banana (25 cm) |
| 24 | 2nd | Viability milestone | Corn (30 cm) |
| 28 | 3rd | Third trimester begins | Eggplant (38 cm) |
| 37 | 3rd | Early term (safe for delivery) | Watermelon (48 cm) |
| 40 | 3rd | Full term, expected due date | Watermelon (50 cm) |
Note: Baby sizes are approximate averages. Individual development varies. All measurements are crown-to-heel from week 20 onward.
Frequently Asked Questions
How many weeks pregnant am I from my due date?
Subtract the number of days until your due date from 280 (total pregnancy days), then divide by 7 to get weeks. For example, if your due date is 90 days away: 280 - 90 = 190 gestational days = 27 weeks and 1 day pregnant. This calculator does the math instantly when you enter your due date.
What trimester am I in based on my due date?
First trimester: weeks 1-12 (more than 196 days remaining). Second trimester: weeks 13-27 (91-196 days remaining). Third trimester: weeks 28-40 (0-90 days remaining). The transition between trimesters is based on completed gestational weeks, not exact days.
Can I calculate my LMP from my due date?
Yes. Your estimated LMP (last menstrual period) date is 280 days before your due date. If your due date is July 15, your estimated LMP was approximately October 8 of the previous year. This assumes a standard 28-day cycle with ovulation on day 14.
Why does my doctor use a different due date than my LMP calculation?
Doctors often adjust the due date based on first-trimester ultrasound measurements, which can be more accurate than LMP-based dating. Ultrasound dating is most accurate between weeks 8 and 13 and can differ from LMP calculations by 5-7 days. If the ultrasound date differs by more than 7 days from LMP, most practitioners will use the ultrasound date.
What percentage of babies are born on their due date?
Only about 4-5% of babies are born on their exact due date. About 80% are born within 2 weeks of the due date (between weeks 38 and 42). First-time mothers tend to deliver slightly later (average 5 days past due date), while subsequent pregnancies tend to arrive closer to the due date or slightly earlier.
How accurate is a due date calculated from LMP?
LMP-based due dates assume a 28-day cycle with ovulation on day 14, which applies to about 10-15% of women. Actual cycle lengths vary from 21 to 35 days, and ovulation can occur on any day between days 11 and 21. This is why first-trimester ultrasound dating (accurate to plus or minus 3-5 days) is preferred for confirming or adjusting the due date.
Can I embed this Pregnancy Week Calculator by Due Date on my website?
Yes. Click the "Embed" button at the top of this page to customize the size, colors, and theme, then copy the iframe code. Paste it into any HTML page, WordPress site, or CMS. It is completely free, requires no signup, and works on all devices. You can also visit our embed guide at calculory.com/services/embed-calculators for more details.
Medical Disclaimer
This calculator is provided for informational and educational purposes only. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any decisions based on these results. Do not disregard or delay seeking medical advice because of information obtained from this tool.
AI Assistant
Ask about this calculator
I can help you understand the pregnancy week calculator by due date formula, interpret your results, and answer follow-up questions.
Try asking
Our AI assistant provides general estimates and advice only. Please consult a qualified healthcare professional before making any health-related decisions.
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