An ultrasound is a very common procedure during pregnancy. It's a non-invasive prenatal test used to ensure that baby is developing properly, to check the baby's estimated size and to see how the baby, placenta and umbilical cord are positioned. At the 20-week ultrasound, it's also an opportunity to learn the baby's sex... if baby is willing to reveal that information to you.

At 20 weeks of pregnancy, the ultrasound is usually done with a special wand that is used on the exterior of the belly to see into the uterus, as seen above. Because the baby is smaller, earlier ultrasounds may be conducted with a "trans-vaginal probe." (To give you a hint what that's like, they use a condom to sheath the probe before inserting it.)

Oh, say can you see?

Ultrasound technology has improved a lot since its advent. If baby cooperates, you can see fingers, toes, spines and even little faces! Also, you may be able to see the baby's anatomy.

Make sure that you make your intentions about finding out the gender (whether you want to or don't) known, so that the ultrasound tech knows not to spill the beans if you don't want them spilled. If you don't tell, then you might accidentally find out ... or worse, you might not be able to find out at all.

You may not be able to recognize much of what you see. On most sonograms, the image is grainy black and white. The transducer -- the part of the machine they rub on your belly -- is moved around almost constantly, and the baby is swimming around, too! (You wouldn't be the first mama-to-be who thought she's expecting not a human baby but an alien one.)

A little cheat sheet: The baby's spine looks a lot like like a string of pearls - lots of little tiny bones in a line. Girl parts look like three lines, while a boy part often appears pretty much as you'd expect. (Be sure not to confuse the umbilical cord and the penis. You might get a surprise at birth if you try to interpret the scan on your own!)

From the front, the baby's face will look a little spooky -- like a skull mask -- from certain angles. But it will seem super cute if you see your little one sucking his thumb in-utero! Amniotic fluid will be one of the darker things on the scan, and the harder tissue -- such as bone -- will appear bright.

Ultrasound prep

Most sonogram imaging specialists will ask you to have a full bladder when you come in. Your bladder, then, acts as a balloon, pushing the uterus up out of the pelvis a bit, which helps the tech visualize the baby, umbilical cord, amniotic sac, placenta and uterus.

But when you're pregnant, keeping a full bladder is easy for them to want, and painful for you to do! If there's not enough urine, you may need to wait until the quantity has increased before they can proceed with the exam. On the other hand, sitting in the waiting room for a half hour or more can be incredibly uncomfortable if you're desperate to pee. When you check in for your appointment, ask if the tech is on schedule to help get a sense of the wait ahead of you. You might need to let off a little fluid in the bathroom (but not too much!) if it gets too difficult.

Cold goo for you

The wand used to administer the ultrasound is slathered with a gooey gel. Unless they have a special warmer on hand, get ready for a quick shock ... the gel is cold. It will remain on your belly throughout the ultrasound. But don't worry, the ultrasound tech will likely clean off as much as possible after the exam, or at least give you the paper towels to do so.

The sound of silence

The ultrasound technician will need to concentrate on finding the right images and measurements for the doctor. (They actually have specific shots they need to capture.) And depending on the tech, that could result in a lot of silence.

"It was nervewracking at first, while the technician was just measuring and not speaking. We were relieved when she told us everything was perfect. We were not uncomfortable at all, just very excited to see our baby growing," said Jacqueline Alvarado of avocadoPR.

So, don't worry. Silence doesn't signal something bad. Chances are, the tech is just concentrating.

Speak up

Not knowing what's coming or what's happening can mean a frustrating situation. But you don't have to sit by quietly.

"For one of my ultrasounds in my first pregnancy, the ultrasound technician turned the screen away for the entire 30 minute process, did not talk to me, and only printed out a picture at the end. I was too shy to say, 'Can I see?' Knowing what I do now, I would have no problem doing that. Speak up, talk to your technician," said Mulligan.

If you can't see the screen, or have questions about the physical structures you're seeing on the ultrasound, just ask. If you don't ask now, you might not have a chance to ask later since techs see many patients every day. (Note that only a physician can diagnose the content of the ultrasound, and many practices, he or she is the only one who is allowed to report your baby's gender.) If you're feeling a little shy, in advance, ask your partner or whoever is with you at the sonogram to advocate on your behalf.

What not to wear

It's best to wear comfortable clothes for your ultrasound appointment, since you will be reclined while the scan is being done. But no matter how comfy your favorite dress is, it's best to leave that at home since you need to expose your belly. Instead, choose comfortable (and machine-washable) separates that can easily be moved away from the belly without leaving you in danger of indecent exposure.

Every ultrasound is different

Even if you've had an ultrasound before, you should know that every ultrasound is different. A variety of factors can influence the experience making it better ... or worse."Your experience can be completely different depending on where the ultrasound takes place (an imaging department vs at a doctor's visit) and unfortunately, what kind of mood your ultrasound technician is in that day -- how much they want to share with you on what they are seeing," said Brenda Mulligan, mother of two.

Usually, the experience should be quite calming. You will be reclining on a padded table or exam chair, the lights will be dimmed and it will be quiet. It's a perfect environment for "meeting" your baby for the first time!

Find out more!

• Accuracy of ultrasounds
• See 100s of scans in the PregnancyAndBaby.com ultrasound gallery!
• More information about prenatal testing and ultrasounds

SheKnows.com editor Nancy J Price, a four-time pregnancy champion and eight-time ultrasound veteran, also contributed to this story.