Our Health Library information does not replace the advice of a doctor. Please be advised that this information is made available to assist our patients to learn more about their health. Our providers may not see and/or treat all topics found herein. There are several ways for your doctor or midwife to figure out how long you have been pregnant. They help you predict when you are likely to have your baby. This is called your due date. The due date is only an estimate of when your baby will be born. Most women deliver within 14 days of their due date.footnote 1 To find out how long you've been pregnant and when your baby is due, your doctor or midwife may: The most common way to calculate your due date is to start with the first day of your last menstrual period (LMP). Add 7 days, and then count backward 3 months. For example, if your last period started on March 20, you would add 7 days to get March 27. Then subtract 3 months to get a due date of December 27. Another way to estimate your due date is to add 40 weeks to the first day of your last period. Another way to find out your due date is by the size of your uterus. When you are about 12 weeks pregnant, your doctor or midwife can feel the top of the uterus (fundus) above your pelvis. After about 18 weeks, the distance between the pubic bone and the fundus (in centimeters) is likely to be about the same as the number of weeks since your last period. At 20 weeks, the fundus will be about as high as your belly button. The size of the uterus is sometimes used to get a rough idea of how far along a pregnancy is. But it's not an accurate way to predict the gestational age of the growing baby (fetus). That's because there are many things that can make the fundus seem higher or lower than it really is. For example, the fetus may be in an odd position. Or you may have a uterine fibroid. If the first two methods can't predict your due date, you may get an ultrasound. Ultrasound tests work well to find out how long you have been pregnant, especially if they are done before 20 weeks of pregnancy.footnote 2 Some doctors always do an ultrasound in early pregnancy. During an ultrasound test, a small device called a transducer is moved back and forth over your belly. The transducer sends out sound waves that bounce off the fetus. The sound waves are converted by a computer into a picture of the fetus. The picture is displayed on a TV screen. Current as of: July 10, 2023 Author: Healthwise Staff Current as of: July 10, 2023 Author: Healthwise Staff Clinical Review Board This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use and Privacy Policy. Learn how we develop our content. To learn more about Healthwise, visit Healthwise.org. © 1995-2024 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.Topic Contents
Pregnancy: Ways to Find Your Due Date
Overview
Last menstrual period
Size of uterus
Ultrasound testing
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Clinical Review Board
All Healthwise education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.
All Healthwise education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.
Our Health Library information does not replace the advice of a doctor. Please be advised that this information is made available to assist our patients to learn more about their health. Our providers may not see and/or treat all topics found herein. Current as of: July 10, 2023 Author: Healthwise Staff Clinical Review BoardPregnancy: Ways to Find Your Due Date
All Healthwise education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.