How to Calculate When a Baby Will Be Born

Expectant parents would love to know exactly when their newest bundle of joy will make his entrance into the world. Having an exact date would help with planning when to finish the nursery, making child-care arrangements for older siblings and simply knowing when the last uncomfortable days of pregnancy will end. But unless you have a planned cesarean section, predicting your baby's precise birth date isn't possible. You can estimate your baby's approximate birthday, though, with some basic information about your pregnancy.

Instructions

1. Identify the date your last menstrual period (LMP) began. Most doctors base a baby's due date on this date, predicting 280 days after the start of Mom's LMP. This date isn't totally reliable, though, since not all women have textbook 28-day cycles, where they ovulate on day 14 of their cycle. If you ovulated later than average, your baby won't start growing as quickly as the doctor assumes, and your due date estimate will be too early.


2. Determine when you ovulated for the most accurate due-date prediction. Methods for identifying ovulation include tracking patterns in your cervical mucus, using ovulation predictor kits, charting your basal body temperature, checking cervix position and noticing cramps or pains on one side of your body that suggest when the ovary released an egg. If you used any of these methods, review any notes you took on your ovulation signs to see which day you likely ovulated. Your estimated due date will be 266 days after you ovulated.


3. Enter the date of your LMP, or the date you think you ovulated, into a pregnancy due-date predictor, such as the one in the Resources section. This calculator allows a more accurate due-date estimate based on LMP, since you can include your average cycle length if you know it. If your cycle is normally longer than 28 days, the calculator will give you a later due date, since you likely ovulated after day 14 of your cycle.


4. Confirm your baby's gestational age with an early first trimester ultrasound. Measuring a baby's crown-to-rump length at seven weeks has an accuracy of plus-or-minus three days. If the ultrasound measurement of your baby agrees with your LMP or suspected ovulation date, your due date is likely accurate. If the ultrasound measurement is very different, you should base your due date on the ultrasound, since it provides an actual measurement of your baby's development.


5. Recognize that due dates can only estimate a baby's likely birthday. According to the American Congress of Obstetricians and Gynecologists, while most babies arrive between 38 and 42 weeks of pregnancy, only 5 percent make their grand appearance on their actual due date. When preparing for your baby's birth, remember childbirth educator Janelle Durham's advice to "be ready for the baby after 38 weeks, but don't be surprised if it doesn't come till 42 weeks."

Source: eHow