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| |    The 'Crown-Rump' Measurement How ultrasound helps to determine fetal age by Laura Jana, M.D., F.A.A.P. reviewed by Marjorie Greenfield, M.D. While some parents can identify the day they conceived, others are less sure. And while obtaining an accurate estimate of the fetus' age is often quite useful to the parents (so that they can start preparing for their new arrival), it can be doubly helpful for the practitioner. That's because many prenatal care recommendations and test interpretations are based on fetal age. One way to determine that age is through the use of ultrasound. When you go in for a fetal ultrasound, you're likely to notice that the person doing the test is taking several measurements of your baby on the ultrasound screen. One of these measurements is taken from the top of the baby's head, or crown, to his bottom, or rump. This measurement is considered to be the most accurate method for determining fetal age early in pregnancy and can be obtained as early as seven weeks. This accuracy is possible because fetuses follow a very predictable growth rate in the first trimester. However, later in pregnancy, a baby's size can vary considerably due to a combination of factors, making measurements (such as the crown-rump length) much less useful in determining fetal age. A typical nine-week-old fetus has a crown-rump measurement of only a couple of centimeters (about the length of a bee) and has a head that is quite large relative to the rest of his body. When your baby's measurement is compared to a set of standard lengths, the result is an estimated age that is considered to be accurate within five days.
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