Ultrasound Safety in Pregnancy

The number of ultrasounds in a pregnancy is dependent on each person’s unique situation. 

Some people need very few ultrasounds during pregnancy, while others, including twins and other high risk pregnancies require many. To date, research has found no association with adverse outcomes for the pregnant person, fetus, or neonate. Because of the limitations of research, the general consensus is to use ultrasounds when medically necessary, and limit their use for non-medical reasons. 

What are some concerned about ultrasound safety? 

• Heat: ultrasound waves heat tissue as they travel through them
  • Temperature increase is minimal (typically less that 1 degree Celsius)
• Vibration & cavitation: as ultrasound waves travel through tissue they cause vibrations which can create gas bubbles
  • Because there is no gas in fetal lungs and intestines this is not thought to be a significant factor for obstetric ultrasounds.

How can we maximize safety? 

• Limit ultrasounds to only when medically necessary.
• Perform ultrasounds over the shortest period of time with the lowest energy output possible.
• Ultrasound should be used with caution in pregnant patients who have a fever (as there is a theoretical risk that together with an already elevated temperature, the heat from ultrasound could cause fetal tissue to reach an unhealthy level).

What is the earliest my pregnancy can be seen on ultrasound?

It’s common to have an ultrasound as early as the first trimester, and many parents are eager to see their babies for the first time. It’s a big moment. So, when can you see that black and white tell-tale ultrasound image you have been dreaming of?

• The beginning of a pregnancy can be seen on ultrasound as early as 4.5-5 weeks gestation (this is based on the first day of your last period)
  • A small sac of fluid, or “gestational sac” can be seen at this time
• Next, a “yolk sac” can be seen on ultrasound between 5-6 weeks
  • This is the first structure that develops within the gestational sac
  • It indicates a pregnancy inside the uterus (as opposed to elsewhere, such as the fallopian tubes)
  • Eventually disappears between 10-12 weeks of pregnancy
• Finally, a “fetal pole” and heartbeat can be seen at 5.5 to 6 weeks
  • You can really visualize an embryo now – your baby is an embryo until about 8 weeks, when the term becomes “fetus” 

Of note, you will likely have a transvaginal ultrasound in the early stages of pregnancy.*

If we don’t see a fetal pole when someone is expected to be 6+ weeks (based on the first day of their last menstrual period) we typically ask them to return for a repeat ultrasound. Irregular periods are common, even in those without a history of this issue, so estimates of gestational age by last menstrual period are not always accurate.

When will I have my first ultrasound?

Most people will have an ultrasound at their first Ob appointment, so your first ultrasound would depend on when you first see your Ob provider. It’s normal to feel many emotions surrounding this event. Many are excited and nervous. 

It is usually recommended that pregnant people have an ultrasound in the first trimester for two reasons. One is to estimate a due date for the pregnancy (the results from the ultrasound will be compared to what we expect to see based on the first day of your last menstrual period). Two is to confirm the location of their pregnancy (rule out an ectopic pregnancy, which is a pregnancy outside of the uterus, usually in the fallopian tube). 

Ultrasounds done earlier in pregnancy (when there is a measurable embryo/fetus with a heartbeat) are almost always more accurate in terms of assigning an estimated due date than those done later in pregnancy. We do not change a due date based on a later ultrasound when a high-quality first trimester ultrasound has been performed. Once in a while, the quality of the first ultrasound is questionable and this will prompt us to reevaluate a due date with a later ultrasound. 

The earliest indication of a pregnancy by ultrasound is at 4.5-5 weeks gestation. If you have an ultrasound this early, it is usually recommended to have follow up later on, because it’s too early to see an embryo and a heartbeat.

If a person does not present for prenatal care until after the first trimester, an ultrasound should be done as soon as possible to accurately date the pregnancy. We also recommend an ultrasound between 18-20 weeks gestation to evaluate baby’s anatomy.

First trimester ultrasound:

Any ultrasound performed in the first trimester (American College of Obstetrics and Gynecology defines this as an ultrasound any time before 14 weeks and 0 days of pregnancy)

• Common indications:
  • Confirm pregnancy in the uterus 
  • Evaluate abdominal pain or vaginal bleeding in pregnancy 
  • Estimate gestational age (and calculate an estimated due date)
  • Do genetic screening tests 
  • Count the number of fetuses 
• The earliest indication of a pregnancy by ultrasound is at 4.5-5 weeks gestation. If you have an ultrasound this early, it is usually recommended to have follow up later on, because it’s too early to see an embryo and a heartbeat

Second trimester ultrasound: 

• Some pregnant people choose to limit the number or ultrasounds performed, especially if they are young and healthy
• It is usually recommended that pregnant people have an ultrasound to confirm the location of the pregnancy (rule out an ectopic at least one ultrasound during pregnancy
  • If only one ultrasound is to be performed, it is recommended that it be done between 18-20 weeks gestation
    • This is a time where ultrasound is still a good tool to confirm an estimated due date/gestational age, and to evaluate the entire anatomy of the fetus/detect significant anomalies

How many ultrasounds will I have in my pregnancy?

It’s important (and recommended) to have at least one ultrasound during a pregnancy, to assess the baby’s anatomy during the 2nd trimester between 18-20 weeks. This anatomy ultrasound is a detailed look at a baby’s entire body. Many pregnant people will have additional ultrasounds as well. The number of additional ultrasounds, and reasons for having them, depend on each person’s individual pregnancy. 

First trimester ultrasounds – most providers and pregnant people opt to have an ultrasound in the first trimester. These ultrasounds may assess for: 

• Pregnancy dating (how far along a pregnancy is, and when the approximate due date is)
• Pregnancy location (especially if there is a complication such as bleeding in the first trimester, an ultrasound may confirm that a pregnancy is in the uterus, and not ectopic – which is a pregnancy outside of the uterus, most commonly in the fallopian tube)
• Markers of genetic abnormalities
  • Nuchal translucency ultrasounds are generally performed between 11-13 weeks, and are part of a genetic screening test
• Baby’s heart rate
• Number of babies (single, twins, triplets)

Third trimester ultrasounds – Not everyone needs an ultrasound in the third trimester, but if you do, it may be related to the following: 

• Baby’s growth
  • If your belly is measuring bigger or smaller than expected, you may have a “growth ultrasound”, often between 28-32 weeks of pregnancy
  • If you have a condition, such as an autoimmune disease, in which there is a higher risk of problems with baby’s growth, your Ob provider may want to follow baby’s growth on ultrasound.
• Placenta location
  • If there has been concern regarding the location of the placenta during pregnancy (sometimes the placenta is close to the cervix, called “low-lying placenta”, and sometimes it’s covering the cervix, called “placenta previa”), this may be re-evaluated in the 3rd trimester
• Baby’s position in the uterus
  • Baby’s position is called his/her “presentation.” It is important to know baby’s position before you go into labor
  • Breech presentation is when the baby’s legs or buttocks are presenting first
  • “Cephalic” or head down – this is the ideal position for a vaginal delivery
  • Transverse – when baby is sideways
  • Variable – if a baby’s position changes frequently 
• Follow up
  • Sometimes a provider recommends follow up on a previously noted issue, such as a baby’s kidney size, or amniotic fluid volume
• Biophysical profile
  • An ultrasound that monitors baby’s well-being if there is any concern, or if baby goes past his/her due date
  • Markers of well-being include: movement, fluid volume, breathing

Transvaginal vs Transabdominal Ultrasound

A transvaginal ultrasound is an ultrasound that is performed using a probe, or wand, that is inserted into the vagina. A transabdominal ultrasound uses a probe externally, on the outside of the abdomen. Both types of ultrasounds are used to visualize a growing baby through sound waves that are transmitted into images, although there are some key differences. 

Timing:

• 1st trimester: Transvaginal may be more accurate in the first trimester, so it is recommended in the earliest stages of pregnancy. 
• 2nd & 3rd trimester: Transabdominal is used more frequently in the second and third trimesters, as the uterus and baby become larger and move into the abdomen from the pelvis. 

Purpose:

• Transvaginal ultrasound are used commonly to:
  • Assess location of the placenta. 
  • Assess cervical length.
  • Confirm a pregnancy is in the uterus. 
• Abdominal ultrasounds are used commonly to:
  • Assess growth of a baby in the 3rd trimester.
  • Confirm baby’s orientation in the uterus (for example, whether or not baby’s head is down). 
  • Specialized ultrasounds if there is a suspected problem with a baby, for example follow up of a baby’s kidneys or heart. 
  • Overall assessment of baby’s well-being (ultrasounds called “biophysical profiles” measure baby’s breathing, movements, and fluid volume).

What is a transvaginal ultrasound?

A transvaginal ultrasound visualizes the uterus and ovaries through the vagina, instead of through the abdomen. A thin wand called a transducer is inserted into the vagina either by an ultrasound technician, or by you (don’t be afraid to ask to insert it yourself if that would make you feel more comfortable). The wand will have a thin cover (sometimes a condom) and gel on it. Sound waves from the transducer create images of your pelvic organs. You will need to have a full bladder during this exam. You may feel some pressure inside the vagina, but you should not feel pain. 

Why would I have a transvaginal ultrasound? 

• A vaginal probe allows us to get closer to better visualize an early pregnancy. An early pregnancy may not be visualized at all with an abdominal ultrasound, and only seen using an internal vaginal probe. 
• A transvaginal ultrasound is useful in confirming that a pregnancy is in the uterus, and not outside of the uterus (ectopic pregnancy). 
• We often use transvaginal ultrasound to accurately measure the length of the cervix. The length of your cervix correlates to your risk of preterm birth. Thus if someone has a higher risk of preterm birth or history of a short cervix, a transvaginal assessment of cervical length is indicated during pregnancy.
  • Transvaginal ultrasound is better able to assess cervical length than abdominal ultrasound 
• To determine how close the placenta is to the cervix
  • If there is concern for the placenta being less than or equal to 2 centimeters from the opening of the cervix, a transvaginal ultrasound can give a closer and more accurate measurement.