How to Use a Due Date Calculator

Using a due date calculator is a quick and reliable way to figure out approximately when your baby will be born. A due date calculator will be the most effective the more accurate the information is that's used in the formula. But remember while the average pregnancy lasts 40 weeks, or 9 complete months, from the first day of the mother's last period, it could last anywhere from 37-42 weeks. Read on for tips on calculating your baby's due date.

Difficulty: Moderately Easy

Instructions

1. 

   It's easiest to predict a due date using a due date calculator, if you know the exact day of conception. Since many women don't know the moment, or even the day, fertilization happened, you should keep track of when your menstrual cycles before you get pregnant. Write down when your period starts and note how many days are in your cycle. Mark the dates on your calendar so that when you get pregnant, you can get a more accurate due date.

2. To manually calculate a baby's due date subtract three months from the first day of your last menstrual period and add 7 days. Let's say your LMP was December 12, 2008 - 3 months would be September 12 + 7 days would be September 19, 2009.

3. An even easier way to figure it out is to use an online due date calculator. These are found on many pregnancy and parenting websites. Simply plug in the LMP information and the length of cycle and your approximate due date will be calculated.

4. If you're unsure of when you conceived and the date of your LMP your doctor will most likely do an internal ultrasound to look at the development of the baby. This will allow the fetus to be measured, giving the doctor an idea of how far along you are and when the baby is due.

Tips & Warnings

• The manual due date formula assumes a woman's cycle length is 28 days.
• Some doctors will give a woman a due date range or "due week," especially if the due date calculator and the baby's development, seen on ultrasound, don't mach up exactly.
• Since due dates are calculated from the first day of your LMP and ovulation doesn't take place for another 11-14 days, you're already considered 2 weeks pregnant on the day of conception.

Resources

• Great Information on Pregnancy for First Time Moms

[lizsto]

on 1/27/2009 Great instructions!

[pamom24]

on 1/26/2009 great info. I used a due date calender to figure out my due date. I was right and the doc office was off by a week.

[Traqqer]

on 1/25/2009 Good suggestion. We used this during our first and second pregnancies.