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Pregnancy Calculators: Which One Should You Use?

You’re going to be a mother! Your estimated date of delivery not only gives you a rough idea when the big day will occur, but also provides your doctor with a valuable piece of information that will help him or her evaluate your baby’s development. Knowing when you are due is also invaluable for planning your lifestyle, diet and exercise program for the duration of your pregnancy, so it’s important to get it right.

Calculating Your Due Date

Due dates are determined using mathematical calculations based on the date of your last menstrual period. Several pregnancy calculators can help you estimate when your baby will be born. Below are a few of the most popular methods of calculation.

Naegele’s Rule

Dr. Franz Naegele, a German obstetrician, devised this widely used formula in the 1800s. It’s based on a 28-day cycle and will be less accurate if your cycle is longer or shorter. The wheel that your OB/GYN uses to give you a due date is based on Naegele’s rule. To calculate your due date:

  • Note the day of your last menstrual period
  • Add seven days
  • Subtract three months
  • Add one year to get your due date.

Don’t want to do the math? Naegele’s rule gives you a date that is approximately 280 days after your last menstrual period.

Couple with home pregnancy test

Wood’s Method

Wood’s method, also called Nichols’ rule, was developed by midwifery professor Carol Wood Nichols. This calculation method is based on an estimated due date of 41 weeks, two days for first babies and 40 weeks, five days for babies after your first. Unlike Naegele’s rule, it can be modified to take into account cycles that are longer or shorter than 28 days. To use Wood’s method:

  • Add 12 months to the date of your last period.
  • Subtract two months and 14 days.
  • The answer is your expected due date if you have 28-day cycles.

 

If your cycles are longer than 28 days:

Subtract the length of your average cycle from 28 days and add that number to the due date estimate. For example, if you have 31-day cycles and your estimated due date using Wood’s method was November 5, 2016:

  • Subtract 28 from 31: The discrepancy between the average cycle and your cycle is 3.
  • Add three days to the estimated due date, which will now make it November 8, 2016.

If your cycle is shorter than 28 days, subtract the number of days from 28, then subtract that number from the due date you calculated using Wood’s method.

Parikh’s Formula

Indian doctor Rakesh Parikh developed his formula after noticing that Naegele’s rule was not very accurate for women with cycles longer or shorter than 28 days. To calculate your estimated due date using Parikh’s formula:

  • Add nine months to the date of your last menstrual period.
  • Subtract 21 days
  • Add the number of days in an average cycle to the date to determine your estimated due date.

Quickening Method

One of the most exciting signs of pregnancy is the day your baby moves for the first time. It’s also used as the basis for one method of calculating pregnancy. To use the quickening method:

  • Note the date you first felt your baby move.
  • Add five months to the date to determine your due date.

Which Method Do the Experts Recommend?

In January, the American Congress of Obstetricians and Gynecologists introduced a new estimated due date calculator app based on recommendations from ACOG, the Society for Maternal-Fetal Medicine and the American Institute of Ultrasound in Medicine.

Basing the due date on the last menstrual period alone is often inaccurate for two reasons. Menstrual cycles tend to vary in length, which affects the date of ovulation, and a woman doesn’t always remember the date of her last period.

The new method uses information from your last menstrual period, but also considers information obtained from an ultrasound conducted during the first trimester to determine your estimated date of delivery. It also takes into account the date that any assisted reproductive technology procedure was performed. Comparing the last menstrual period with information from an ultrasound allows obstetricians to offer much more accurate estimates.

Why Is an Accurate Due Date Important?

An accurate due date is important for several reasons:

  • It gives doctors the information they need to schedule time-sensitive tests, such as the alpha-fetoprotein blood test, which is used to detect spina bifida and other birth defects.
  • It ensures that your doctor assesses the correct milestones at crucial points in your baby’s development.
  • It is an essential factor in planning for birth. For example, if you need a caesarean section, your doctor will use your due date in determining the best time to schedule the procedure.
  • It allows you to make plans, such as coordinating your maternity leave with your employer.
  • It permits you to focus on other important aspects of your pregnancy. Once your due date is established, you can begin to think about doing everything you can to ensure you have a healthy pregnancy, such as starting a fitness routine and making good nutritional choices.

Good Nutrition Is Essential for a Healthy Baby

Your growing baby benefits from the foods you eat. Although you won’t need to double the amount of food you eat, you will need to add about 300 to 450 calories to your average daily calorie count. Making sure you eat the healthiest foods is an excellent way to offer your baby the best possible start in life.

A healthy pregnancy includes calcium for bone, teeth, heart and nerve function and development; folic acid, which decreases the risk of neural tube deficits; iron to help you feel less fatigued; and many other nutrients. Not sure how to change your diet to help your baby thrive? A registered dietitian can help you create the perfect pregnancy diet.

He or she will not only help you create the best diet for your unique needs, but will also inform you which foods and beverages should be avoided or limited during pregnancy. If you have unpleasant pregnancy symptoms, such as morning sickness, your dietitian can recommend food choices that will help you manage this difficult period of pregnancy.

If you just got a positive result on your pregnancy test, now is the perfect time to make a few important changes to your diet. The registered dietitians at LifeStyle Medical Centers can help you ensure that both you and your baby enjoy a healthy, happy pregnancy. Call us today to make an appointment.

 

Sources:

American Congress of Obstetricians and Gynecologists: ACOG Reinvents the Pregnancy Wheel

American Congress of Obstetricians and Gynecologists: Download the ACOG App

ACOG: Method for Estimating Due Date

Cleveland Clinic: Calculating Your Estimated Due Date

Chronicle of Young Scientists: Erroneous Calculation of the Expected Date of Delivery by Medical Professionals,  July/September 2011

Cleveland Clinic: Good Nutrition During Pregnancy for You and Your Baby

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