Early ultrasound due dates have a 1.2-week margin of error. After 20 weeks of pregnancy, you should not adjust your anticipated due date based on an ultrasound since it will be less accurate. Also, keep in mind that this is a projected due date; the great majority of women do not have their infants on the day they are due.
The accuracy of an ultrasound for determining due dates declines as the pregnancy advances. The margin of error rises to plus or minus two weeks between 18 and 28 weeks of gestation. The ultrasound may be three weeks or more off in estimating a delivery date beyond 28 weeks.
Ultrasounds can also be inaccurate in predicting small birth weights, low birth weights, preterm births, and fetal deaths. Tests cannot determine genetic abnormalities or diseases that cause no physical changes until after birth.
An ultrasound examination uses sound waves to create images of organs inside the body. The ultrasound technician moves a transducer through the uterus to view the fetus. Each time the transducer is moved it generates another image. These images are converted into pictures that allow the doctor to see detailed views of the fetus's anatomy not otherwise visible with the naked eye. Modern scanners can produce images in multiple dimensions, including from several angles, which allows them to capture subtle variations in fetal shape not apparent when viewing the fetus face-on.
Due to limitations of resolution, an ultrasound cannot detect all abnormalities. For example, it cannot distinguish between two identical fetuses or one baby and one extra piece of tissue. It also cannot identify many minor anomalies at birth that are visible on prenatal photographs taken around the same time as the ultrasound.
Late-Pregnancy Ultrasounds It is usual for each ultrasound to anticipate a different due date during the pregnancy. After that, it is usually only plus or minus one week. A later ultrasound may show your baby earlier than expected.
If you think that your ultrasound due date is incorrect, call your doctor's office and ask them what date they would recommend for your next appointment. Most offices will tell you that it is okay to come in sooner rather than later, but some people have their babies even earlier than what the doctors estimated, so it's good to know your limits as far as waiting goes before scheduling another test.
Ultrasound Tests An ultrasound measures the thickness of tissue (ultrasound scans) or fluid (Doppler tests) inside your body. This information can help your doctor diagnose certain conditions, such as diabetes, hypertension, kidney disease, heart problems, fetal abnormalities, and more.
You may need an ultrasound examination before you are scheduled to give birth if your doctor suspects a problem with your baby's growth or development. For example, if your baby was born prematurely, your doctor might want to see how much his or her brain has grown since birth.
Evidence shows that ultrasounds can forecast your due date more precisely than your previous menstrual period—but only in the first and early second trimesters (until roughly 20 weeks). Later estimates are less accurate, with a 4-week margin of error for each step up in accuracy.
The sound wave is transmitted into the body, where it is reflected by different tissues depending on their composition. The echoes from these reflections are collected by the scanner and converted into pictures of the fetus on which doctors can make measurements. Modern scanners use high-frequency sound waves that pass easily through the body wall and into the fetus. They can visualize details of organ structure and blood flow not seen with lower frequency instruments.
An ultrasound screening test can detect many problems before a woman knows she is pregnant. It can also show how far along you are in pregnancy, including determining fetal gender if possible. This information can help doctors plan care for you and your baby. An ultrasound exam should be done as soon as possible after finding out that you are pregnant. This allows enough time for all of the structures involved in the pregnancy to show up on the scan.