Due Date Calculator

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due date calculator
Last Menstrual Period (LMP)Conception Date
Estimated Results:
Your Estimated Due Date is
function toggleInputs(){var method=document.getElementById('calc_method').value;var cycleRow=document.getElementById('cycleRow');var dateLabel=document.getElementById('dateLabel');if(method==='conception'){cycleRow.style.display='none';dateLabel.innerHTML='Date of Conception:';}else{cycleRow.style.display='table-row';dateLabel.innerHTML='First Day of Last Period:';}}function calculateDueDate(){var method=document.getElementById('calc_method').value;var dateInput=document.getElementById('input_date').value;var cycleLength=parseInt(document.getElementById('cycle_length').value);if(!dateInput){alert('Please select a valid date');return;}var startDate=new Date(dateInput);var dueDate=new Date(startDate);if(method==='lmp'){var adjustment=cycleLength-28;dueDate.setDate(startDate.getDate()+280+adjustment);}else{dueDate.setDate(startDate.getDate()+266);}var today=new Date();var diffTime=Math.abs(today-startDate);var diffDays=Math.floor(diffTime/(1000*60*60*24));var weeks=Math.floor(diffDays/7);var remainingDays=diffDays%7;var options={weekday:'long',year:'numeric',month:'long',day:'numeric'};document.getElementById('eddValue').innerHTML=dueDate.toLocaleDateString(undefined,options);document.getElementById('gestationValue').innerHTML='Current Gestational Age: '+weeks+' weeks, '+remainingDays+' days';if(document.getElementById('show_details').checked){var t1=new Date(startDate);t1.setDate(t1.getDate()+91);var t2=new Date(startDate);t2.setDate(t2.getDate()+189);var details='

First Trimester Ends: '+t1.toLocaleDateString()+'

Second Trimester Ends: '+t2.toLocaleDateString()+'

Third Trimester Ends: '+dueDate.toLocaleDateString()+' (Due Date)

';document.getElementById('trimesterDetails').innerHTML=details;document.getElementById('trimesterDetails').style.display='block';}else{document.getElementById('trimesterDetails').style.display='none';}document.getElementById('calculatorAnswer').style.display='block';}

Using the Due Date Calculator

A due date calculator is an essential tool for expectant parents to estimate when their new addition will arrive. While only about 4% of babies are born exactly on their estimated due date, having a target date helps with medical planning, nursery preparation, and monitoring developmental milestones.

To get the most accurate results, enter the following information into the tool above:

Calculation Method
Choose between "Last Menstrual Period" (the most common medical standard) or "Conception Date" if you know exactly when fertilization occurred.
Start Date
The first day of your last period or the date of conception.
Cycle Length
The average number of days between your periods (standard is 28). This helps adjust for longer or shorter cycles.

How It Works: The Math Behind the Date

The standard medical calculation for pregnancy duration is based on a 40-week (280-day) gestation period. Our due date calculator utilizes Naegele's Rule, which is the gold standard used by OB-GYNs worldwide.

EDD = LMP + 280 Days + (Cycle Length – 28)

Here is how the variables work:

  • LMP: Most pregnancies are dated from the first day of the last menstrual period because it is a concrete date that most women remember.
  • Gestation: Human pregnancy lasts approximately 266 days from conception, but since conception usually happens 14 days after the LMP, the "pregnancy" is dated at 280 days.
  • Cycle Adjustment: If your cycle is 32 days instead of 28, you likely ovulated 4 days later than average, so the calculator adds those 4 days to your due date.

Calculation Example

Example: A woman had her last period start on January 1st. She has a longer-than-average menstrual cycle of 32 days.

Step-by-step solution:

  1. Input LMP = January 1st
  2. Standard Gestation = +280 days (leads to October 8th)
  3. Cycle Adjustment = 32 days – 28 days = +4 days
  4. Final Calculation: October 8th + 4 days
  5. Estimated Due Date = October 12th

Common Questions

Can my due date change?

Yes. While the due date calculator provides an excellent estimate based on your dates, an ultrasound (specifically a first-trimester dating scan) is often more accurate. If the ultrasound measurement differs by more than 7 days from the LMP date, your healthcare provider may adjust your official due date.

What if I have irregular cycles?

If your cycles vary wildly (e.g., 25 days one month and 40 the next), the LMP method may not be accurate. In these cases, doctors rely almost exclusively on early ultrasound measurements to determine the age of the fetus.

Is pregnancy really 9 months?

Actually, a full-term pregnancy is closer to 10 months (40 weeks) when counting from the LMP. Most people refer to it as 9 months because fertilization occurs about two weeks into that 40-week window.

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