Most babies are positioned headfirst by the end of the third trimester.
Determine fetal position before delivery. Your health care provider might use ultrasound to guide needle placement during certain prenatal tests, such as amniocentesis or chorionic villus sampling. If you're bleeding or having other complications, an ultrasound might help your health care provider determine the cause. An ultrasound can help your health care provider screen for some birth defects. An ultrasound can help evaluate the placenta and amniotic fluid around the baby. Too much or too little amniotic fluid - the fluid that surrounds the baby in the uterus during pregnancy - or complications with the placenta need special attention. The placenta provides your baby with vital nutrients and oxygen-rich blood. Study the placenta and amniotic fluid levels. Ultrasound can be used to monitor your baby's movement, breathing and heart rate. Your health care provider can use ultrasound to determine whether your baby is growing at a normal rate. If your health care provider suspects a multiple pregnancy, an ultrasound might be done to confirm the number of babies. Knowing the baby's age can help your health care provider determine your due date and track various milestones throughout your pregnancy. Determine your baby's gestational age. A fetal ultrasound can help your health care provider detect a pregnancy outside of the uterus (ectopic pregnancy). 
Some fetuses develop outside of the uterus, in the fallopian tube.
Confirm the pregnancy and its location. Your health care provider might use fetal ultrasound to: A specialized or detailed exam is done when an anomaly is suspected based on your history or other prenatal exam results. Examples include the evaluation of fetal growth and the estimation of amniotic fluid volume. However, the timing of this ultrasound might be altered for reasons such as obesity, which could limit visualization of the fetus.ĭuring the second and third trimesters, limited ultrasound evaluation might be needed when a specific question requires investigation. This exam is typically done between weeks 18 and 20 of pregnancy. In the second or third trimester a standard ultrasound is done to evaluate several features of the pregnancy, including fetal anatomy. Ultrasound can also be used for first trimester genetic screening, as well as screening for abnormalities of your uterus or cervix. 
It might be used to confirm or rule out a congenital heart defect.įirst trimester ultrasound examination is done to evaluate the presence, size and location of the pregnancy, determine the number of fetuses, and estimate how long you've been pregnant (gestational age). This exam provides a detailed picture of a baby's heart. It can provide details about a baby's blood flow. A Doppler ultrasound measures slight changes in the ultrasound waves as they bounce off moving objects, such as blood cells. This type of ultrasound is sometimes used to help health care providers detect facial abnormalities or neural tube defects. This exam provides a two-dimensional display of three-dimensional data. In this situation, a more detailed evaluation can provide additional information about the abnormality. This type of exam might be needed in specific circumstances, such as when a fetal abnormality is known or suspected. Various other types of transabdominal ultrasounds are available, including: A transabdominal fetal ultrasound is done by moving a transducer over your abdomen. This type of ultrasound also might be done if a transabdominal ultrasound didn't provide enough information. Transvaginal ultrasounds are used most often during early pregnancy. With this type of fetal ultrasound, a wandlike device called a transducer is placed in your vagina to send out sound waves and gather the reflections.
There are two main types of fetal ultrasound exams: If a problem is suspected, a follow-up ultrasound or additional imaging tests, such as an MRI, might be recommended. If your pregnancy remains uncomplicated, the next ultrasound is typically offered during the second trimester, when anatomic details are visible. The first fetal ultrasound is usually done during the first trimester to confirm the pregnancy and estimate how long you've been pregnant.
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