I always find it interesting to see the broad range of thoughts and desires that patients have. This is very apparent in prenatal care, and it comes up often with ultrasound. There are patients who wish they could have an ultrasound at every appointment and (less commonly) those who don't want to have any ultrasounds at all. I do review the topic of ultrasounds at every patient's first appointment in the pregnancy, but I commonly hear, "When's my next ultrasound?" and sometimes, "Is that ultrasound necessary?" as the pregnancy progresses.
It's generally important to have at least one ultrasound in the pregnancy that can assess, if nothing else, any anatomic or growth issues that could impact the wellbeing of the fetus(es) and the location of the placenta. This type of ultrasound is typically performed mid-way through the pregnancy at about 20 weeks. This assessment will allow the safest management of the pregnancy going forward and planning for timing, mode, and location of delivery. If there is a congenital defect that would require surgical intervention for the infant soon after delivery, for instance, delivery optimally should happen at a site that allows speedy access to neonatal surgery. If there is a severe anomaly that is incompatible with life, some patients may choose termination of the pregnancy. If the placenta is growing over the opening of the cervix, there will be recommended activity restrictions to avoid bleeding complications, and a C-section will be recommended if this issue persists. For the patients that are hoping to avoid ultrasounds completely, I counsel them that I can't ensure that I am doing my best to keep their pregnancies safe without this type of ultrasound.
Before this, many patients will also have at least one ultrasound in the first trimester. An early ultrasound is often done to assess 1. the number of fetuses 2. with cardiac activity 3. at what gestational age 4. in what location. Knowledge of multiples will guide some pregnancy recommendations. Identification of an abnormally-progressing pregnancy/early pregnancy loss allows for appropriate counseling on management options. Due date and gestational age are typically calculated based on a patient's last menstrual period, but some patients do not have regular periods or are unsure of the timing of their last period, and the ultrasound can provide a more reliable estimation of the due date. Sometimes pregnancies grow outside the uterus (ectopic pregnancies), and this can be life-threatening.
An ultrasound may be done near the end of the first trimester as part of genetic screening depending on which, if any, testing a patient selects. This ultrasound looks at the entire pregnancy but focuses on a fluid pocket at the back of the neck of the fetus (nuchal translucency). If this is thicker than expected, it indicates an increased risk for Down Syndrome or other chromosomal abnormality as well as other anatomic concerns.
After a normal ultrasound around 20 weeks, an overall healthy patient without any complications in the pregnancy does not require any more ultrasounds. When concerning findings are identified on that ultrasound, follow-up ultrasounds will likely be scheduled for monitoring or reevaluation. If a routine office exam suggests that the uterus is bigger or smaller than expected based on gestational age of the pregnancy, an ultrasound might be done to survey fetal growth and the amount of fluid surrounding the fetus. The position of the fetus at term or when a patient goes into labor is not always able to be confirmed by physical exam, and an ultrasound can determine if the fetus is head-down, breech, or some other position to most safely plan delivery. There are some chronic and pregnancy-related medical conditions that may impact the growth and development of the fetus and increase the risk of stillbirth, and continued assessments of growth and wellbeing via ultrasound may be recommended in the third trimester.
A patient who underwent recommended ultrasound monitoring in one pregnancy may not necessarily require the same types of ultrasounds in a subsequent pregnancy. On the flip side, someone who had just one standard ultrasound during pregnancy might need increased monitoring in a future pregnancy. Each pregnancy can come with different circumstances and risks.
Why not just do more ultrasounds during every pregnancy if a patient wants them? Like anything in medicine, we hope to limit testing to circumstances when it is medically necessary. We don't just order every possible lab test and perform a full-body CT scan on every person every year of their lives. If we did, we would probably identify some conditions early in their development, but that benefit would be outweighed by significant potential risks. An ultrasound uses sound waves that contact all of the various parts of a pregnancy and then bounce back for conversion into images. Although we currently have no evidence that this is harmful to a pregnancy, we cannot rule out potential risks completely. Ultrasound also cannot definitively identify all diagnoses and has a margin of error in terms of growth measurements that widens more and more as a pregnancy progresses - simply put, an ultrasound can be very wrong. Further ultrasounds also increase the time and costs of medical care, which are important considerations given the high costs associated with healthcare in this country.
As always, every person should talk to their prenatal care provider about what makes sense for them. Whether ultrasounds are fun or nerve-racking for you, everyone should be on the same page working toward the best possible outcomes for the pregnancy.