To give a short answer first – as long as you stick to the number of scans suggested by your doctor, then you and your baby are safe.
Now for the longer answer.
Like many scientific and technological things that are introduced in the last few decades, the long term effects of ultrasound scanning on your baby is, unfortunately, not known. However, since it has been prevalent in the last 10+ years, we can safely infer that there has not been any evidence of the scan causing harm to the baby. Usually, a pregnant woman with no complication is advised to have at least five scans during her pregnancy. Millions of women give birth to healthy babies after these five scans – so it is definitely safe. But more importantly, a scan helps the doctor assess your baby’s heartbeat, growth rate, position of the baby, the amount of amniotic fluid inside your womb and the position of the placenta. It has also proven to be a great diagnostic tool to detect birth defects such as Down’s syndrome in unborn babies. So if you weigh the benefits against the potential “unknown” risks, then ultrasound scans definitely get a green flag. This is the reason we say that as long as you have only as many number of scans as your doctor has advised (which is most often – 5), then it should be okay
Although it is not risky, it is good to understand how the scan works and what are the potential risks involved.
When the radiologist moves the ultrasound probe over your abdomen, it emits high frequency waves, which bounce off the baby. These bounced off waves help forming an image of the baby. Similarly the radiologist is able to get an image of the placenta, the cervix, the uterus and so on. These high frequency waves can also produce heat. But, do not panic. The intensity of the ultrasound used during pregnancy scan is very less, so the heat generated is only 1° Celsius or below.
That said, medical researchers claim that if the scan results in the temperature of the body tissues being scanned (in this case the fetus) goes up by even 4° Celsius, then it can be very harmful. For instance, if the normal body temperature is 37° Celsius, then it should not go higher than 41° Celsius. Again, no need to panic. The temperature of the tissue spiking up that much during an ultrasound scan is very unlikely. This is because:
All these are applicable for 2D, 3D and 4D scanning.
More number of scans are advised when it is a high risk pregnancy. For instance, your gynecologist will demand more scans if you:
In these cases, the doctor advises more scans because, like we said before, the benefits of the scan outweighs the risk involved. It is important for the doctor to ensure at all times that your pregnancy is viable and for this, the doctor might need more number of scans. If you are worried, or have more questions on why you require more-than-usual number of scans, then please talk to your doctor. Ensure that there is a medical reason to do more scans.
Apart from this, ensure you get your scan done by well-trained and experienced radiologists sitting in reputed clinics with good infrastructure. For more about ultrasound scans, refer to 3D & 4D Ultrasound Scans During Pregnancy.
Hope you have a safe pregnancy and delivery!