At 20 weeks, the baby is completely formed and just needs to grow and mature. Even if you’ve never seen an ultrasound before, you should be able to recognize the shape of the baby and see it’s heart beating. Despite all the buzz about color 3D images of the baby, where it looks like a ‘real’ baby with the skin on, your ultrasound will mostly be the traditional black and white pictures you may have seen over the last couple of decades. This is because 2D is still the best way for the doctor and sonographer to see the structures below the baby’s skin, looking inside the baby where the anatomy lives. Learn more about the differences of 2D versus 3D and 4D. Below is a video of a 20 week 2D ultrasound.
To better understand what you are seeing, note that the very top of the image is the mother’s skin (where the probe is) and the bottom of the image is the mother’s muscles and spine behind the uterus. Think of the image as a top-down slice taken from the top (mother’s skin), through the uterus, through the placenta, fluid, and baby, with the bottom of the image being at the mother’s back. Parents quickly realize they are easily able to see right through the baby to the bones, ribs, etc., and many of them ask if the baby has skin yet or remark that the baby must still be transparent. The baby DOES have skin and is not transparent (think of a premature baby—it definitely is not transparent- slightly translucent skin maybe, but you certainly can’t see the bones and organs underneath). The ultrasound beam is able to “see” through the fetus, just as it ‘sees’ through the mother’s skin to the structures behind.
It will also help you to understand what the various shades of white/gray/black represent. On ultrasound, fluid is BLACK. You can see that the baby is moving surrounded by fluid (amniotic fluid) and there are areas of black within the baby as well, which are other types of fluid. Cerebrospinal fluid (which you can see in the head) and blood in the heart and blood vessels show up as black on ultrasound. On ultrasound, any WHITE you see is bone and cartilage. The different shades of gray represent different densities of soft tissue such as the placenta, and muscles and organs within the baby. In the video above, you can see that there is a large patch of smooth gray above the baby and black of the amniotic fluid. This is the placenta, which lies just below the surface of the mother’s skin. This is known as an anterior placenta, which is a great location, however, it may affect how well the mother is able to feel the baby. Click for more on feeling fetal movements.
In the video above, notice how you don’t clearly see the arms and legs when you are able to see the beautiful profile of the face. This is because the ultrasound beam is being transmitted in a single plane, or a thin slice going from the top down. Since the slice is going straight along the fetal forehead, nose, lips, neck, and chest, the arms and legs are lying to the right and left side of the slice and aren’t visible. However, as you watch the baby move, you can see parts of the arms and legs coming in and out of the picture as the baby moves.
Is the baby’s gender going to be obvious? Again, because ultrasound is a single plane of data, you’re not able to see the whole baby ‘naked’ like you would if it were lying in front of you. In order to see the gender, specific views need to be taken between the legs, and good sonographers can usually avoid letting you see that view if you don’t want to know. Many people are confused by what they are seeing even when they are shown that specific view between the legs, so don’t assume you’ll easily figure out what it is. Click for more on how to determine gender with ultrasound.
What does 3D or 4D look like at 20 weeks?
First of all, 4D is simply a series of single 3D images so that there is the appearance of motion. The 4th ‘D’ is time. Because 20 week fetuses are still skinny without much body fat, and their heads appear large compared to their thin limbs and body, they can look pretty scary. It is very hard to get a really cute 4D video at 20 weeks. The other important thing to remember when you look at any 3D/4D image is that it is a computer-constructed image, not a glimpse at the real thing. As such, the computer can make some odd assumptions as it is trying to render a pleasuring surface to the baby and weird bulges and lumps appear. In addition, the image is being ‘directed’ by the sonographer, who uses the 3D/4D software to try and adjust the area in focus, rotate, expand, or contract the field of view to create the most pleasing image. Meanwhile, the baby is moving and wiggling, making the job of getting a good window of view even harder.
Below is a 20 week 4D video, and you can see how the sonographer is trying to chase the baby, rotate the picture to catch the baby at a pleasing angle, and cut away data (the placenta, cord, and hands) that is blocking view of the baby. Sometimes you’ll notice parts of the skull get cut out which makes it look as though there is a gaping hole in the head, the nose sometimes looks funny, the baby’s mouth is opening at times giving it an odd appearance, and is generally is not a video you’d want to show off to the family at holidays. This video is unfortunately pretty typical of the type of images we get on 20 week fetuses. Read more on the limitations of 3D and 4D.
