Pregnancy Week Calculator

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pregnancy week calculator
Last Menstrual Period (LMP)Conception DateKnown Due Date
Results:
function calculateResult(){var m=parseInt(document.getElementById('month').value);var d=parseInt(document.getElementById('day').value);var y=parseInt(document.getElementById('year').value);var cycle=parseInt(document.getElementById('cycle').value)||28;var method=document.getElementById('calc_method').value;var showSteps=document.getElementById('steps').checked;if(isNaN(m)||isNaN(d)||isNaN(y)||m12||d31){alert('Please enter a valid date (MM/DD/YYYY).');return;}var inputDate=new Date(y,m-1,d);var today=new Date();var lmpDate,dueDate;if(method==='lmp'){lmpDate=new Date(inputDate);var adjust=cycle-28;dueDate=new Date(lmpDate.getTime()+(280+adjust)*24*60*60*1000);}else if(method==='conception'){lmpDate=new Date(inputDate.getTime()-14*24*60*60*1000);dueDate=new Date(inputDate.getTime()+266*24*60*60*1000);}else{dueDate=new Date(inputDate);lmpDate=new Date(dueDate.getTime()-280*24*60*60*1000);}var diffMs=today-lmpDate;var diffDays=Math.floor(diffMs/(1000*60*60*24));var weeks=Math.floor(diffDays/7);var days=diffDays%7;if(diffDays<0){alert('The date entered results in a future pregnancy start. Please check your dates.');return;}var trim="";if(weeks<13){trim="First Trimester";}else if(weeks<27){trim="Second Trimester";}else{trim="Third Trimester";}document.getElementById('progressText').innerHTML="You are "+weeks+" weeks and "+days+" days pregnant.";document.getElementById('dueDateText').innerHTML="Estimated Due Date: "+dueDate.toDateString();document.getElementById('trimesterText').innerHTML="Current Trimester: "+trim;if(showSteps){var stepsHtml="Calculation breakdown:
";stepsHtml+="Estimated LMP: "+lmpDate.toDateString()+"
";stepsHtml+="Total Days elapsed: "+diffDays+" days
";stepsHtml+="Cycle adjustment: "+(cycle-28)+" days
";document.getElementById('detailSteps').innerHTML=stepsHtml;document.getElementById('detailSteps').style.display='block';}else{document.getElementById('detailSteps').style.display='none';}document.getElementById('answer').style.display='block';}

Using the Pregnancy Week Calculator

A pregnancy week calculator is an essential tool for expectant parents and healthcare providers to track the developmental milestones of a pregnancy. Most pregnancies last approximately 40 weeks (280 days), and tracking your progress helps you prepare for each trimester. This tool calculates your current week of pregnancy, your estimated due date (EDD), and which trimester you are currently in.

To get the most accurate results, you should provide the date of your last menstrual period, although you can also calculate based on conception or a known due date provided by an ultrasound.

Last Menstrual Period (LMP)
The first day of your most recent period. This is the standard starting point for medical pregnancy dating.
Average Cycle Length
The number of days between the start of one period and the start of the next. The default is 28 days.
Conception Date
The specific date when fertilization occurred. This is usually about 14 days after the LMP in a 28-day cycle.

How It Works

The pregnancy week calculator uses Naegele's Rule as its primary logic, with adjustments for cycle length variations. Because most women do not know the exact moment of conception, the medical community dates pregnancy from the first day of the last menstrual period (LMP). This means that during weeks 1 and 2 of "pregnancy," you are not actually pregnant yet; your body is preparing for ovulation.

Due Date = LMP + 280 Days + (Cycle Length – 28 Days)

  • Gestational Age: The age of the pregnancy measured from the LMP.
  • Fetal Age: The actual age of the growing baby, usually 2 weeks less than gestational age.
  • Trimesters: The 40 weeks are divided into three blocks: 0-13 weeks (1st), 14-26 weeks (2nd), and 27-40+ weeks (3rd).

Calculation Example

Example: A user has a Last Menstrual Period starting on January 1st, 2024, and has a standard 28-day cycle. They are checking their progress on March 15th, 2024.

Step-by-step solution:

  1. Identify LMP: January 1, 2024
  2. Determine Days Elapsed: January (31) + February (29 in 2024) + March (15) = 75 days
  3. Convert to Weeks: 75 days / 7 = 10 weeks and 5 days
  4. Calculate Due Date: Jan 1 + 280 days = October 7, 2024
  5. Result: The user is 10 weeks and 5 days pregnant, in the 1st Trimester.

Common Questions

Why is the pregnancy week calculator different from my ultrasound?

The calculator uses dates, which assumes regular ovulation. An ultrasound measures the physical size of the fetus (crown-rump length). If you ovulated later or earlier than day 14 of your cycle, the ultrasound is often considered more accurate for dating in the first trimester.

Can I use this if I have irregular cycles?

Yes, though it may be less precise. You should enter your average cycle length. If your cycles vary wildly, it is best to rely on a dating ultrasound performed by a medical professional.

How many weeks is a full-term pregnancy?

While 40 weeks is the "due date," a pregnancy is considered "full term" starting at 39 weeks. Delivery between 37 and 38 weeks is considered "early term."

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