Obstetrical Ultrasound

What is an obstetrical ultrasound?

Obstetrical ultrasounds, or prenatal ultrasounds, are a safe and non-invasive test that use sound waves to create a real time image. These images give us information about the health and development of a fetus. Ultrasound also gives information about a woman’s uterus, ovaries, cervix, the placenta, and the blood flow supporting the fetus.

It is typical to have 3-4 ultrasounds, but you might have more or less depending on your unique pregnancy. The most commonly ordered ultrasounds are called the obstetrical series, which include a dating ultrasound, nuchal translucency/first trimester screen, and the detailed anatomy ultrasound.

Gel is placed on the end of the ultrasound wand (transducer), and a trained ultrasound technologist (sonographer) moves it across the abdomen. The sound waves pass through the gel and send information back to the computer, which creates the images. These images are then read by a specially trained doctor. There is no radiation used during an ultrasound. In certain situations, a vaginal ultrasound might be recommended as part of your appointment. Your health care team will discuss this with you, and it will only be done with your consent.

***For more information about specific obstetrical ultrasound exams please see our blog posts regarding the Dating Ultrasound, The Anatomical Scan, and the Third Trimester Ultrasound***

First trimester (up to 14 weeks) ultrasound:

These ultrasounds are done up until 13 weeks and 6 days of pregnancy. During the first trimester most people can expect to have 1-2 ultrasounds ordered. These ultrasounds can give us information such as:

• Confirmation of pregnancy
• Early pregnancy complications such as ectopic pregnancy
• Pregnancy due date and gestational age
• Fetal growth, development, and heart rate
•  Number of fetuses (twins, triplets.etc)
• Potential congenital and genetic conditions
• The health of the uterus, ovaries, cervix and placenta

The most common first trimester ultrasounds are the dating ultrasound and Nuchal Translucency ultrasound (NT).

Dating Ultrasound (Between 6-10 weeks gestation)

At the dating ultrasound, we look to see if the pregnancy is growing in the right place, how many fetuses there are, the due date, and the fetal heart rate. Sometimes it is too early to see some of these things, and you may be asked to return for another ultrasound at a later date. We usually try to do this ultrasound around the 8^th week of pregnancy. To guess the 8^th week of pregnancy, we count 8 weeks forward from the first day of the last menstrual period.

Advanced First Trimester Screen (Between 11 weeks-13 weeks 6 days gestation)

The advanced first trimester screen includes the Nuchal Translucency (NT) ultrasound and tests for the risk (likelihood) of a fetus having Trisomy 21 (Down syndrome), Trisomy 18 (Edwards Syndrome) and Trisomy 13 (Patau Syndrome). The NT ultrasound measures the fluid at the back of the fetus’s neck, called the nuchal fold. When this space is bigger, it can sometimes mean there might be a genetic or chromosomal condition. We will also look for the nasal bone. The absence or presence of the nasal bone and the nuchal fold measurement are combined with some medical information about the pregnant woman, and a calculation is made for how likely or unlikely the fetus is to have these conditions.

During this ultrasound we also have an in depth look at the fetus’s developing anatomy for any other visible conditions, the placenta, and the woman’s pelvic organs.

It is important to remember that this ultrasound cannot diagnose any conditions, it can only give the likelihood of these conditions. If you have a positive screen, counselling with our maternal fetal medicine specialist will be organized. If you wish to confirm a diagnosis, our team will help to guide and set up further testing and appointments with the right specialists.

Second Trimester Ultrasounds (Between 14-27 weeks gestation)

These ultrasounds are done between 14 weeks and 27 completed weeks of pregnancy.  In the second trimester, ultrasound is most often looking closely at the growth and development of the fetus’s physical structures (anatomy) such as the bones and organs.

Detailed/Anatomy Ultrasound

this ultrasound is done around the 20^th week of pregnancy (18-22 weeks). This is usually the longest appointment, because the sonographer will look at many things including the fetus’s bones and internal organs, placenta and umbilical cord, the fluid around the fetus, and the woman’s uterus, ovaries, and cervix. If there are any concerns with the way the fetus or the pregnancy is developing, it can often be seen at this ultrasound. Sometimes, all of the fetal anatomy can’t be seen in one appointment because of the position of the fetus. If this happens, you may be asked to return for another appointment to make sure everything can be seen clearly.

At this ultrasound you can find out the sex of the fetus if you wish. You can be told at your appointment, or the information can be provided to you in a sealed envelope.

Third Trimester Ultrasounds

These ultrasounds are done at 28 weeks of pregnancy and greater. In your third trimester, you may have an ultrasound for some of the following reasons:

• Monitoring the baby’s size and growth
• Monitoring the amount of amniotic fluid around the baby
• Monitoring any changes in your cervix
• Checking what position the baby is in
• Looking at the location and health of the placenta