By Mathew Miller, Sharon Perkins

As excited as new dads are for your second-trimester ultrasound, the actual event can sometimes be a bit of a letdown. Reading ultrasounds is an art, and unless the ultrasonographer is really patient about pointing things out, you may be unsure of whether you’re viewing the baby’s head or his tush.

Much depends on the direction the baby’s facing. You may get a somewhat frightening straight-on face shot, which looks far more like the creature from Alien than any relative of yours, or you may get a front-on foot view that looks like nothing more than five round balls.

You may be happy to know the baby has five toes on each foot, but that’s usually not the main information parents-to-be want.

How pregnancy ultrasounds measure growth

First and foremost, your medical practitioner wants to know that the baby is growing as he should. Some of the measurements taken to check for normal growth include

  • The length of the longest leg bone, called the femur

  • The head circumference

  • The head diameter, called the biparietal diameter

  • The abdominal circumference

Comparing these measurements to standards assures your practitioner, and you, that the fetus is growing as he should.

How pregnancy ultrasounds check for genetic markers

Genetic markers indicate an increased risk of congenital problems, but as with the blood tests, genetic markers merely indicate the risk potential; they don’t diagnose the disease. Some ultrasound markers are known as soft markers because they’re often misinterpreted and not as diagnostic as other signs. Soft markers may also be transient and no longer seen in later ultrasounds. Following are genetic markers, including soft markers:

  • Bowel abnormalities: Around 12 percent of Down syndrome babies have gastrointestinal defects that may be spotted on ultrasound.

  • Cardiac defects: Around 50 percent of Down syndrome babies have cardiac defects, which may be visible via ultrasound.

  • Kidney abnormalities: Dilated kidneys, missing or small kidneys, and other anomalies may indicate genetic disorders.

  • Missing nasal bone: Failure to see the nasal bone or a shortened nasal bone on ultrasound may indicate Down syndrome.

  • Polyhydramnios: An increased amount of amniotic fluid may be associated with congenital defects.

  • Shortened arm and leg bones: Children with congenital abnormalities often have arms and legs that are shorter than normal.

  • Thickness of the skin on the back of the neck: Called nuchal translucency, thicker-than-normal neck skin indicates an increased risk of Down syndrome.

How to determine the baby’s sex on ultrasound… or not

Whereas the ultrasonographer’s priority is looking for information that shows the baby is growing properly, your consuming interest during the second-trimester ultrasound may be the baby’s sex. Ultrasonographers who do prenatal ultrasounds are well versed in not blurting out the baby’s sex and usually ask whether you want to know.

Most generically use he or she to avoid calling the baby it if you don’t want to know, so don’t assume anything by the choice of words if you’ve requested that you not be told. You can feel legitimately concerned if the ultrasonographer starts using the term they, though!

Ultrasounds generally aren’t done just to satisfy parental curiosity but rather to catch any potential problems early on. If the baby’s sex can’t be determined in the first ultrasound and you absolutely must know in advance, your insurance will likely require you to pay out-of-pocket for another ultrasound.

If you had your heart set on a girl and it’s as plain as the nose on your face, even to your untrained eyes, that a little boy is on the way (or vice versa), remember that feeling a twinge of disappointment is normal and okay. Try to keep it to yourself and concentrate on what you’re probably seeing — a healthy, normally developing child.

Basics of 3-D or 4-D pregnancy ultrasounds for dads

Newer 3-D and 4-D ultrasounds can pick up far more detail than 2-D images. In some cases, newer ultrasound techniques allow practitioners to filter out extraneous detail and add color and shadows to images for a far more realistic look at your baby’s features and activities.

Your medical practitioner may only suggest a 3-D ultrasound if she suspects there’s a problem from a standard ultrasound. If you’ve had a standard ultrasound that was perfectly normal, you can still take advantage of improving ultrasound technology by going to the nearest commercial facility offering nonmedical ultrasounds for a photo shoot.

Just keep in mind that the people running and working at these centers may not be medically trained professionals.

Ask for credentials so you know that the person doing your ultrasound actually knows what he’s looking at. Also, talk to your medical practitioner about any concerns she may have about this type of ultrasound before signing up.