Hello, everyone! Welcome back to the Santa Clarita Valley (SCV) Birth Center blog. Today, we are discussing ultrasounds when they should and maybe should not be used in pregnancy. Let’s jump in!

Ultrasound at 30 weeks

Over the weekend, someone reached out to me on Facebook asking about an ultrasound that was done on their family member who was 30 weeks pregnant. The doctor mentioned that the baby was measuring a bit small, but the weight was exactly as expected.

Keep in mind, this was a late-term ultrasound, which is anything after 24 weeks for a low-risk, normal singleton (meaning only one child) pregnancy with no known issues. So, it got me thinking: why do we do these late ultrasounds and how much can we really rely on the information they provide? And ultimately, what does the evidence say?

Do we need late ultrasounds for size checks?

Here at the SCV Birth Center, SCV’s premier birth center, we’re all about the evidence. We want to let the evidence direct how we do things. One of the reports that influences our evidence-based approach for ordering ultrasounds is a report from the National Library of Medicine, National Institute of Health (NIH).

According to the report, late-term ultrasounds after 24 weeks of gestation, for low-risk pregnancies without any anomalies or medical concerns, diagnostically cannot tell us really how big the baby is. And there’s no clear benefit to doing multiple ultrasounds. In fact, these late ultrasounds are associated with a slight increase in cesarean rates. They don’t have any potential positive psychological effect and there is limited data on how it affects childhood outcomes.

When we do order late ultrasounds

At our birth center here in Santa Clarita, we only refer clients for late ultrasounds if there’s a noticeable difference in size measurements on multiple prenatal visits. So, if a mother measures three centimeters (or more) larger or smaller than expected, we’ll recommend an ultrasound to assess the baby’s growth and other factors like amniotic fluid levels.

The way we measure as midwives is fascinating by the way. As midwives, we use our hands to perform what is called Leopold’s maneuver, in which we measure from the mother’s pubic bone to fundal height – fundus is the top of the uterus. So we measure from the pubic bone to the top of the uterus because we know that measurement is within centimeters of the true measurement.

And there’s a great reason we measure in centimeters – the measurement from pubic bone to fundus should be roughly equal to how many weeks pregnant the mother is. So, for example let’s say a mother is at 30 weeks gestation, we’re going to measure pubic bone to fundus and it should be between 28 and 32 centimeters. The design works quite well.

What do the ultrasound results actually mean?

So, if we’ve sent a mother in for a late ultrasound because her size measurements were small or large for multiple prenatal visits, we will then take a look afterward to gather more information and to better understand what’s going on.

We look at measurements like the bi-parietal diameter (diameter of baby’s head), head circumference, and abdominal circumference to determine if there are any concerns. For the head circumference versus the abdominal circumference, those really should be pretty symmetric.

If those start to get asymmetric, especially if the abdominal circumference is much smaller than the head circumference, that’s an indicator we really look at for a small-for-gestational-age (SGA) baby or intrauterine growth restriction (IUGR).

What to do if your baby appears to be growing slowly

If we see this indication, we would then follow up to learn more about the mother’s nutrition and other possible contributing factors.

  • How much coffee is the mother drinking, or how much caffeine is she intaking?
  • What is the mother’s caloric intake?
  • How much exercise is the mother getting?
  • Can we make any tweaks to the mother’s nutrition and diet?
  • Is the mother diabetic, insulin dependent, or not diet controlled gestational diabetic?
  • And others

Once we identify some potential contributing factors to the baby’s small size or slow growth, we will make some tweaks and then follow up with another ultrasound. We want to determine what does and does not have an effect on the baby’s growth.

But here’s the other important part: if someone tells you, or you get a report that says, that your baby is measuring a bit small without any specific criteria or assessment, it doesn’t hold much weight. All babies grow at different rates, and the ultrasound measurements can’t accurately determine the actual mass of the baby.

During ultrasounds, technicians take specific measurements for things like head circumference, abdominal circumference, and femur length. Then they estimate – they make an educated guess – the baby’s weight based on those measurements, providing an average value. So, if you receive a report saying your baby is measuring a few days smaller or larger, it doesn’t necessarily mean there’s a problem or that your due date needs to be adjusted.

Step back: how much do you need early term ultrasounds?

Let’s backtrack a little bit. Back in the day, ultrasounds weren’t as common as they are now. We didn’t have a gazillion billion ultrasounds, and they certainly weren’t done at every single prenatal visit. But times have certainly changed.

Let’s start by discussing the number of ultrasounds that are considered appropriate and why they are performed. In midwifery practices and out-of-hospital birth settings, ultrasounds are not conducted at every visit. Midwives are skilled at using their hands to measure and assess the position of the baby (remember Leopold’s maneuver?). This allows them to determine the growth and position without relying solely on ultrasounds. However, in many obstetrical practices, multiple ultrasounds are performed, often starting with an eight-week ultrasound to confirm the pregnancy’s location and viability.

At around ten weeks, another ultrasound is conducted to ensure the baby is still viable. Subsequently, a 12-week ultrasound is commonly performed to provide reassurance and check on the baby’s growth. In some practices, ultrasounds are done at each visit, which can really rack up the bills.

However, in midwifery practices like ours, once we can detect the fetal heartbeat externally using a Doppler, which is a form of ultrasound technology focused on sound (no visuals), the chances of miscarriage drop to less than 2 percent. This allows us to confirm the viability of the pregnancy without the need for frequent ultrasounds.

When to consider early ultrasounds

At this point, this is when we ask our clients who are considering early ultrasounds:

  • Why are we doing it?
  • What are we going to learn?
  • How is it going to help us move forward?

If you have unclear dates, you were not tracking your cycles, you don’t know when you ovulated, or you’re really just quite confused about when your baby might be due, that’s a really valid reason to do an early ultrasound.

But, if you have good dates, you’ve been planning this pregnancy, you have a pretty good idea when your period was, when you ovulated, when there was intimacy for conception, you’re not symptomatic, you’re not bleeding, and you’re not having any discomfort, then we can go along with those dates. We don’t feel that it’s necessary to do an early ultrasound in this situation. Not to mention, those early ultrasounds can be quite invasive, and sometimes they have to be done with an internal wand. That means a transvaginal ultrasound.

This involves inserting a wand into the vagina to obtain a clear view of the uterus during early pregnancy. Not the most comfortable experience, to say the least! However, transvaginal ultrasounds are necessary when the uterus is still deep inside the pelvic cavity and a regular transabdominal ultrasound may not provide accurate results due to the inability of ultrasound waves to penetrate bone (yes, your pubic bone!).

But fear not, as the pregnancy progresses and the uterus rises above the pubic bone, a transabdominal ultrasound becomes sufficient for most purposes. There are only a few circumstances where a transvaginal ultrasound might still be needed later in pregnancy, such as when there are concerns about the location of the placenta, like placenta previa. However, these instances are relatively rare.

What ultrasounds are good for

So, what should you consider when deciding whether to have an early ultrasound? Always ask yourself why you are doing it and what you hope to learn from it.

If you’re going for genetic testing or an anatomy scan later in pregnancy, those ultrasounds can provide valuable information. The anatomy scan, typically performed between 19 and 21 weeks, allows for a comprehensive assessment of the baby’s development and provides vital insights into their health.

However, it’s important to remember from earlier that, as the pregnancy progresses, the accuracy of ultrasounds in estimating size and weight decreases. Late-stage ultrasounds, such as those done around 36 weeks to estimate fetal weight, often have a margin of error of plus or minus two pounds and may not provide reliable information about the baby’s actual size. So, it’s important not to get too fixated on those numbers and instead focus on the overall well-being of both the baby and the mother.


While early ultrasounds can be valuable in certain circumstances, such as determining gestational age or assessing potential complications, they may not always be necessary for everyone. Trust your instincts, rely on accurate dates if available, and consider the benefits and drawbacks of each ultrasound. And most importantly, remember that the size of your baby doesn’t dictate your ability to have a safe and healthy childbirth experience.

Babies come in all shapes and sizes, and as long as the measurements are symmetric, within a reasonable range, and the other information around the mother’s circumstances are relatively normal, there’s usually no cause for alarm.

Always ask questions and seek clarification from your care provider. It’s your right to understand the information they provide and make informed decisions. If something doesn’t feel right or you’re unsure about a procedure, don’t hesitate to speak up and ask for further explanation. You have the power to advocate for yourself and your baby throughout your pregnancy journey.

I hope this information has provided some clarity on the topic of early ultrasounds and their role in prenatal care. Remember, pregnancy is a unique journey for each individual, and the decisions you make should align with your personal circumstances and preferences.

Stay informed, ask questions, and trust in your body’s incredible ability to nurture and bring forth new life. Wishing you a joyful and fulfilling pregnancy experience ahead!

If you have any further questions or concerns, feel free to reach out to us. We’re here to support you every step of the way.

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