If you are around 20 weeks pregnant, you're probably getting ready for your first big ultrasound.

An ultrasound is a non-invasive prenatal test done by a medical professional with a special wand called a transducer. A technician uses the transducer on the exterior of the belly to ensure your baby is developing properly and see how the placenta and umbilical cord are positioned.

But that's not all that happens! That's right; at 20 weeks, you are also usually able to learn your baby's sex. For many, it's an exciting point in pregnancy.

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Here's what to expect during your 20-week ultrasound.

What does the ultrasound show?

Ultrasound technology has improved a lot since its advent. If Baby cooperates, you can see fingers, toes, spines and even a little face. Also, you may be able to see the baby's anatomy. Make your intentions known about finding out the sex (whether you want to or don't), so that the ultrasound tech knows not to spill the beans. If you don't tell, then you might accidentally find out — or worse, you might not be able to find out at all.

The ultrasound works by sending high-frequency sound waves that are used to produce images of what's inside you. You may not be able to recognize much of what you see, but the technician is trained to interpret the images. On most sonograms, the image is grainy black and white. The transducer — the part of the machine they rub on your belly (wand) — 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.)

Ultrasound cheat sheet

Spine

Some moms think the baby's tiny spine looks a lot like a string of pearls — lots of little tiny bones in a line. It reminds me of the spine of dinosaur skeletons you see in natural history museums.

Girl or boy?

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.

Little face

Waiting for the first glimpse of Baby's face? 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 or her thumb in-utero.

More to look for

Amniotic fluid will be one of the darker things on the scan, and the harder tissue — such as bone — will appear bright.

Preparing for the ultrasound

There really isn't much to preparing for the ultrasound. Usually, the sonogram imaging specialists will ask you to have a full bladder (but then, what pregnant woman doesn't feel like she has a full bladder?). Depending on how long you have to sit in the waiting room, you may want to bring a bottle of water so you can unload and refill while you wait so you aren't too uncomfortable.

More: Mom-to-be sees dead grandmother in ultrasound image

Why does your bladder need to be full? When your bladder is full, it acts as a balloon, pushing the uterus up out of the pelvis a bit, which helps the tech see the baby, umbilical cord, amniotic sac, placenta and uterus.

Up next: What to wear

Originally published September 2012. Updated December 2016.

What to wear

The technician will need to access your whole belly. Wear a loose-fitting top that can be easily moved up and pants with enough give to be pulled down a bit. Also, do not wear a dress or you'll be in your skivvies for the exam.

Cold goo for you

The wand used to administer the ultrasound is slathered with a gooey gel that allows the wand to easily glide over your skin. 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 annotate.) And depending on the tech, that could result in a lot of silence. "It was nerve-wracking 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 Avocado PR.

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

Speak up

While you do need to let the technician concentrate on doing her job, not knowing what's happening can be frustrating. Don't be afraid to ask questions about what you are seeing (or if you can't see). "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 Brenda Mulligan, mother of two.

There won't be a second chance to find out everything you want to know about what you are seeing. If you don't ask now, you probably won't have a chance to ask later since techs see many patients every day and won't be able to answer your questions. Feeling shy? Enlist your partner or whomever goes with you to advocate on your behalf.

Note: Only a physician can diagnose the content of the ultrasound, and at many practices, he or she is the only one who is allowed to report your baby's sex.

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 Mulligan.

Just relax. 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.

More: There's something really fishy about these moms' ultrasound photos

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

Before you go, check out our slideshow below.

Image: St. Clair Hospital