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| |    What Ultrasound Can Show You During Pregnancy Images obtained by ultrasound by Marjorie Greenfield, M.D. reviewed by Laura Jana, M.D., F.A.A.P. In pregnancy, ultrasound can be helpful in determining how many weeks old the fetus is (otherwise known as gestational age), how the baby is growing, if the fetal anatomy seems normal, and the number of babies in the uterus. It can reveal the location and condition of the placenta. Ultrasound can also be used to look at some aspects of the mother, such as uterine fibroids and cervix length. While some practitioners use it routinely, others use it only if there are specific indications. Determining gestational age with ultrasound Early in the pregnancy a measurement called crown-rump length is used to determine gestational age. Later, the leg bone (femur) length and a measurement across the head (biparietal diameter or BPD) are used. Measurements of the fetus are most precise in determining gestational age in the first trimester (accurate to within one week), less exact in the second trimester (within two weeks) and least accurate as the baby gets larger in the third trimester (can be off by three weeks.) By the third trimester, different babies grow at very different rates, so size is much less accurate for determining fetal age. What ultrasound reveals early in the first trimester By five and a half weeks gestation, ultrasound can show that the embryo is in the uterus (or not, in the case of ectopic/tubal pregnancy.) By seven weeks, ultrasound can usually show the heart beating, which indicates that miscarriage is unlikely. Early first trimester ultrasound can also be used to look for twins. What ultrasound can reveal later in pregnancy Depending on technical factors like position of the baby etc., ultrasound can provide images of the baby that help us to assess his welfare. By 18 weeks gestation, ultrasound can often see the baby's:
What ultrasound cannot do Ultrasound looks at physical characteristics of your baby. When a clear view is obtained, the sonographer can often be certain of findings, like how many chambers are in the heart, or the intactness of the spine. Sometimes there is uncertainty in what is seen; maybe the normal structure was not seen well, or something out of the ordinary was present. This may require a consultation with a specialist and/or a repeat ultrasound later in the pregnancy when the organs are larger. Ultrasound looks indirectly at the major body parts and can miss important findings. In addition, although ultrasound can sometimes pick up a hint of a chromosome problem, it cannot identify all babies with genetic problems. Families often equate ultrasound with finding out the sex of their baby. When the baby is well visualized, ultrasounds are clear enough now that even girl-parts can be identified with good accuracy. However, sometimes sex cannot be determined. It is important to realize that ultrasounds are fairly expensive, and are not usually offered just to see fetal sex, but rather to gather information useful for the medical care of the mother and baby.
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