Second trimester ultrasound

During a typical ultrasound done between 16 and 20 weeks, your practitioner will:

Check your baby's heartbeat

To make sure it's normal, your practitioner will measure the number of beats per minute.

Measure your baby's size

Your practitioner may measure your baby from crown to rump, across the skull, along the thighbone, and around the abdomen in an attempt to make sure he's about the size he should be for his age.
If she has any concerns about how your baby is growing, she may order a series of ultrasounds over time to check his progress.

Check the location of the placenta

If the placenta is covering the cervix (placenta previa), it can cause painless but severe bleeding (especially later in the pregnancy) that can be risky for you and the baby. If your practitioner detects this condition, she'll most likely order a follow-up scan in four to six weeks to see if the placenta has moved up out of the way.

Assess the amount of amniotic fluid in the uterus

Too much or too little amniotic fluid can signal a problem with the baby. Most likely everything will be fine, but your practitioner will probably want to monitor you more closely with regular ultrasounds for the remainder of your pregnancy.

Check the baby for structural abnormalities

Your practitioner will look closely at your baby's basic anatomy, including the heart, spine, stomach, kidneys, bladder, and umbilical cord to determine if they're developing properly. If you've had any suspicious results from a multiple marker screening or if there's any other cause for concern, the technician will do a more thorough (level II) scan to check for certain markers that may indicate a birth defect or chromosomal abnormality including Down syndrome.

Try to determine your baby's gender (if you want)

If you'd like to find out your baby's gender, you usually can at this point, unless your child's hand is covering his or her genitals, for example, during the scan.

Ultrasound may be done at other times during your pregnancy to check on the baby's condition or as part of other tests, such as the nuchal translucency test or amniocentesis as a follow-up to the multiple marker test.

During second trimester you may have a sonogram done at 16 - 17 weeks to screen for soft makers that cause chromosomal abnormalities and your doctor may also order blood test(s).