As it turns out, most women only need a couple of ultrasounds during pregnancy. Read on to learn more about these important prenatal scans.

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During a pregnancy ultrasound, your doctor or a skilled technician uses a plastic transducer to transmit high-frequency sound waves through your uterus. These sound waves send signals back to a machine that converts them into images of your baby.

Ultrasounds can provide your doctor with lots of valuable information—for example, they can monitor your baby's growth, detect abnormalities, predict your due date, determine whether you're carrying multiples, show the position of your placenta, and indicate the sex of your baby. 

To prepare you for these important scans, we broke down the most common types of pregnancy ultrasounds week by week and when you should expect them. 

Early Pregnancy Ultrasound (6-8 Weeks)

Your first ultrasound, also known as a baby sonogram, might take place when you're six to eight weeks pregnant. But not every woman will get this scan; some doctors only conduct it for certain high-risk pregnancy conditions like bleeding, abdominal pain, and history of birth defects or miscarriage. 

An early pregnancy ultrasound may be done transgvaginally so doctors get a clearer picture of your baby. In this case, the OB-GYN will place a thin wand-like transducer probe—which transmits high-frequency sound waves through your uterus—into your vagina. The sound waves bounce off the fetus and send signals back to a machine that converts these reflections into a black and white image of your baby.  

At six weeks' gestation, it's possible to see the baby's heartbeat. Your practitioner will also predict your baby's due date, track milestones, determine the number of babies in the womb, and see whether you have an ectopic pregnancy.  

Dating Ultrasound (10-13 Weeks)

Those who forgo the six to eight week ultrasound might have a "dating ultrasound" around weeks 10 to 13 of pregnancy. This gives parents the same type of information: due date, your baby's "crown-rump length" (measurement from head to bottom), the number of babies in the womb, and fetal heartbeat

Nuchal Translucency Ultrasound (14-20 Weeks)

Between 14 and 20 weeks, you may also have a nuchal translucency (NT) test to check for Down syndrome, heart defects, or other chromosomal abnormalities, says Joanne Stone, M.D., a professor of obstetrics, gynecology, and reproductive science at Mount Sinai School of Medicine in New York. Women should consider getting it if their screening test revealed a potential problem, they're 35 or older, or they have a family history of certain birth defects.

In a nuchal translucency screening, the doctor will use an ultrasound to gauge the thickness at the back of the baby's neck (they'll also measure hormones and proteins with a blood test). A thicker neck may indicate an increased risk for birth defects like Down syndrome and trisomy 18. 

Anatomy Scan (18-20 Weeks)

This detailed pregnancy ultrasound, which generally happens between weeks 18 and 20 in the second trimester, lasts 20 to 45 minutes if you're having one baby and longer if you're having multiples. It's the most thorough check-up your baby will have before they're born.

During the anatomy scan, the doctor will check your baby's heart rate and look for abnormalities in their brain, heart, kidneys, and liver, says Jane Chueh, M.D., director of prenatal diagnosis and therapy at Lucile Children's Hospital Stanford, in Palo Alto, California. They'll count your baby's fingers and toes, check for birth defects, examine the placenta, and measure the amniotic fluid level. And they'll probably be able to determine your baby's sex, although it's not a slam dunk; an experienced tech gets it right more than 95 percent of the time. (If you don't want to know your baby's sex, let them know ahead of time.) 

Third Trimester Ultrasound

Many parents-to-be don't need an ultrasound in the third trimester. But if your pregnancy is considered high-risk—for example, if you have high blood pressure, bleeding, low levels of amniotic fluid, preterm contractions, or are over age 35 —your doctor may perform in-office, low-resolution ultrasounds during your prenatal visits for reassurance, says Dr. Chueh. You'll also get a follow-up ultrasound if your cervix was covered by the placenta at your 20-week scan.

Doppler Ultrasound

A regular ultrasound uses sound waves to produce images; this one bounces high-frequency sound waves off circulating red blood cells to measure blood flow and blood pressure. The test will determine if your baby is getting enough blood.

Other Pregnancy Ultrasounds 

Your provider may also conduct other pregnancy tests that require ultrasounds. These might include chorionic villus sampling (CVS) or amniocentesis. Fetal echocardiograms, which show the baby's heart rate and detect abnormalities, also use ultrasound technology.

How Many Ultrasounds During Pregnancy Are Safe?

An ultrasound is considered safe for both you and your baby when it's used for medical purposes. Although ultrasounds require no radiation, only a trained professional who can interpret the results with accuracy should perform them. Your technician should be schooled in obstetrical ultrasound, preferably at a center accredited by the American Institute of Ultrasound in Medicine

Some medical practices now offer 3D (high quality and lifelike) and 4D (moving picture) ultrasounds, which may help doctors detect certain fetal abnormalities and birth defects. However, these types of exams are also available at fetal portrait studios in places like shopping malls. Experts discourage these "keepsake" ultrasounds since untrained personnel may give out inaccurate information, says Michele Hakakha, M.D., an OB-GYN in Beverly Hills and author of Expecting 411: The Insider's Guide to Pregnancy and Childbirth.

Plus, although ultrasounds are safe in medical settings, they might heat tissues or produce bubbles (cavitation) during use. Experts aren't sure about the long-term effects of heated tissues or cavitation when ultrasounds are conducted outside of a medical need with non-trained professionals. "Therefore, ultrasound scans should be done only when there is a medical need, based on a prescription, and performed by appropriately-trained operators," according to the FDA in a December 2014 statement.

How Much Does An Ultrasound Cost?

Ultrasounds aren't cheap procedures; they can cost hundreds or thousands of dollars, depending on your location and provider. Most health insurance plans will cover the cost of the ultrasound (at least partially) if it's for medical purposes. Always ask your insurance provider if you're unsure about expenses.