The expected date of your baby's arrival is probably the biggest piece of news you'll take away from your first checkup. It's the date you'll memorize and engrave on your heart. Unfortunately, it's probably not the date your baby will arrive. The due date is nothing more than an estimate that gives you and your doctor an idea of when to expect your bundle of joy. Because the exact date of conception is almost impossible to pinpoint, and because babies have their own time schedule for arrival, you should use this date only as an indicator of when you might be due.
Your health-care provider will determine when your baby is due by using a handy little "due-date calculator" that figures out the date by simply targeting your last menstrual period. To do this calculation yourself, try one of two formulas:
If your periods are a predictable 28-day cycle, your calculated due date should be fairly accurate. But if your cycles are longer or shorter than 28 days, your actual due date might fall before or after the projected date.
As your pregnancy progresses, your health-care provider will double-check the many factors that indicate the age of a fetus. She'll note the growing size of the uterus. She'll mark the time you first feel movement and when the heartbeat is detected. If these dates don't match up with your projected due date, your doctor might suggest that you take an ultrasound test. The ultrasound will show the size of the fetus and give a very good indication of its age.
Excerpted from The Complete Idiot's Guide to Pregnancy and Childbirth © 2004 by Michele Isaac Gliksman, M.D. and Theresa Foy DiGeronimo. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.
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