What is an ultrasound?

A prenatal ultrasound (also called a sonogram) is a noninvasive diagnostic test that uses sound waves to create a visual image of your baby, placenta, and uterus, as well as other pelvic organs. It allows your healthcare practitioner to gather valuable information about the progress of your pregnancy and your baby's health.

During the test, an ultrasound technician (sonographer) transmits high-frequency sound waves through your uterus that bounce off your baby. A computer then translates the echoing sounds into video images that reveal your baby's shape, position, and movements. (Ultrasound waves are also used in the handheld instrument called a Doppler that your practitioner uses during your prenatal visits to listen to your baby's heartbeat.)

You may have an ultrasound at your practitioner's office at 6 to 10 weeks confirm and date the pregnancy. Or you may not have one until the standard midpregnancy ultrasound between 16 and 20 weeks. That's when you may learn your baby's sex, if you like. (The technician will probably present you with a grainy printout of the sonogram as a keepsake.)

You may also have a sonogram as part of a genetic test, such as the nuchal translucency test, chorionic villus sampling, or amniocentesis, or at any other time if there are signs of a problem with your baby. You'll have more frequent ultrasounds if you have diabetes, hypertension, or other medical complications.

On the other hand, if you're having a low-risk pregnancy, you might not be offered an ultrasound at all. In fact, the American Congress of Obstetricians and Gynecologists recommends ultrasounds only when there's a specific medical reason. According to the March of Dimes, about 70 percent of pregnant women in the United States have an ultrasound.

What information will my midpregnancy ultrasound provide?

Parents talk about finding out their baby's sex

During a typical midpregnancy sonogram, with the help of a sonographer, your practitioner will:

Check your baby's heartbeat

To make sure the heartbeat is normal, your healthcare provider measures the number of beats per minute.

Measure your baby's size

The sonographer will measure your baby across the skull, along the thighbone, and around the abdomen to make sure he's about the size he should be for his age. If this is your first ultrasound and your baby is more than two weeks bigger or smaller than he should be, it's likely that your due date is off and you'll be given a new one. If your practitioner has any concerns about how your baby is growing, she'll order one or more follow-up ultrasounds to check his progress.

Check to see if there's more than one baby

By now you'll probably know if you're carrying twins or higher multiples. Most women pregnant with multiples measure large in their first trimester and have an ultrasound at that point to confirm the number of babies.

Check the location of the placenta

If the placenta is covering the cervix (placenta previa), it can cause bleeding later in the pregnancy. If your practitioner detects this condition, she'll most likely order a follow-up scan early in your third trimester to see if the placenta is still covering the cervix. In the meantime, don't panic! Only a small percentage of placenta previas detected on an ultrasound before 20 weeks are still posing a problem when the baby is due.

Assess the amount of amniotic fluid in the uterus

If the sonogram shows that you have too much or too little amniotic fluid, there may be a problem. You'll have a complete work-up to see if the cause can be identified, and your practitioner may want to monitor you with regular ultrasounds.

Check the baby for physical abnormalities

Your practitioner will look closely at your baby's basic anatomy, including his head, neck, chest, heart, spine, stomach, kidneys, bladder, arms, legs, and umbilical cord to make sure they're developing properly. If you've had any suspicious results from a multiple marker or first trimester or if there's any other cause for concern, the technician will do a more thorough (level II) scan to check for signs of a birth defect or Down syndrome.

Try to determine your baby's sex

If you'd like to find out whether your baby's a boy or a girl, you usually can at the midpregnancy ultrasound (16 to 20 weeks), unless, for example, your child's hand is covering his genitals during the scan. In some cases, it's important for your practitioner to know your baby's sex – for example, if the baby is thought to be at risk for certain congenital conditions.

Let your technician know if you don't want to know your baby's sex so that she doesn't spoil the surprise during the test.

Why would I need an ultrasound during my first trimester?

Some practitioners routinely recommend an ultrasound at 6 to 9 weeks to confirm and date the pregnancy, but others will only do one if they have reason to suspect any of the following:

Miscarriage

If you have vaginal bleeding early in your pregnancy, your practitioner may be concerned about miscarriage and want to schedule an ultrasound to check on your baby. The baby's heartbeat should be clearly visible by 6 weeks of pregnancy (assuming a 28-day menstrual cycle). If you can see the baby's heart beating on the sonogram after 7 weeks, your chances of continuing with the pregnancy are greater than 97 percent.

If you don't see the heart beating at this point, don't despair. It may be that your cycle is longer than average and your baby is younger than your practitioner figured. She may want to try again in a week.

Ectopic or molar pregnancy

Vaginal bleeding (as well as other symptoms) can also indicate an ectopic or molar pregnancy. In the case of an ectopic pregnancy, which occurs when the embryo isn't in the uterus, your practitioner will try to determine where the embryo is. In a molar pregnancy, in which there's an abnormal placenta and usually no viable baby, the ultrasound may show what looks like a tiny cluster of grapes where the baby should be.

Uncertain gestational age

If you're unsure of the date you started your last menstrual period or the length of your cycle, your practitioner may suggest an ultrasound at 6 or 7 weeks to see how far along you really are.

Since all fetuses are about the same size in their early weeks, your practitioner can usually determine your baby's gestational age (and thus your approximate due date) by taking certain measurements. When you're between 7 and 13 weeks, measuring from the crown of your baby's head to his rump will allow her to determine the baby's age within three or four days.

Multiple gestation

If you're measuring large (or if you've had fertility treatments), you may be pregnant with twins or higher multiples. Your practitioner will order an ultrasound to see how many babies you're carrying.