If you haven’t had an ultrasound before, this is how a standard one goes.

You’ll be asked to arrive at the office with a full bladder. You may find this uncomfortable, but a full bladder eliminates pockets of air between your uterus and bladder, which helps create clear ultrasonic images.

When the technician brings you in the room (and in most places, your partner can come in with you), the tech will have you lie on an exam table. He or she will then apply a special gel to your abdomen (which may be a little cold–just what you need on a full bladder). However, the gel helps to conduct sound waves so the ultrasound sends and receives these waves clearly for a good picture.

The tech will then rub a transducer back and forth over your belly. The transducer looks like a handheld scanner that a cashier uses to check the price of that heavy bag of dog food you don’t want to lift from the bottom of the shopping cart. As the smooth end of the transducer is rubbed over your stomach, it bounces sound waves off of dense body parts like bone and muscle. Then, these sound waves are converted into black-and-white or gray images on a monitor.

There is no pain involved (other than that full bladder) and ultrasounds are perfectly safe for you and your baby.

The technician will take your baby’s measurements and explain what you see on the screen and point out the face, toes and what not. Don’t worry if you can’t “see” your baby. Ultrasound images can be hard to make out if you are not a trained technician. They print certain images for for your doctor and you will probably be given copies of some of the images as well.

Once your obstetrician sees your ultrasound, there are a few different ways he or she might interpret it to measure your baby.

a) The Crown-Rump Length – measures the baby from the top of her head to the bottom of her butt and and is very accurate method for estimating the due date based on the baby’s size. This type of measurement can be made somewhere between 7 to 13 weeks of pregnancy.

One important thing to know about this: once your doctor determines the baby’s due date based on the crown-rump length, this due date will not change, even if another ultrasound is done. For instance, if another scan done several weeks later says that one should have a new due date which is further away, one should not normally change the date but should rather interpret the finding as that the baby is not growing at the expected rate.

b) Head Size. This measurement, called the biparietal diameter, measures the diameter from one side of the head to the other. You have to have this measurement taken early in the pregnancy but after 13 weeks. However, taking the head measurement late in pregnancy is unreliable. Early in pregnancy, the size of healthy babies fall into a narrow range. As a baby gets closer to delivery, however, heads sizes of healthy babies vary a lot more.

c) Thigh Length. The doctor looks at the measurement of the longest bone in the body, called femur length, which reflects the growth of the fetus. The same criteria are used here as with a head size measurement: must be taken early in the pregnancy and is only really used to tell how old the baby is.