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  • Nathan Riley, MD

Beloved Holistics Radio Episode 77 - Ultrasound in Pregnancy

Updated: Aug 3

"Beauty is a manifestation of secret natural laws, which otherwise would have been hidden from us forever." - Johann Wolfgang von Goethe



PB #175, Published December 2016 (Reaffirmed 2020)


Five Pearls

1. US may be harmful if overutilized.

2. Important uses include pregnancy dating/viability, fetal growth, and amniotic fluid assessment.

3. US pregnancy dating is most accurate in the 1st trimester. If significant discordance exists between US dating and LMP dating, it may be appropriate to adjust due date.

4. Fetuses at risk for FGR should be monitoring by serial growth ultrasound. It's not recommended to repeat growth ultrasound more frequently than q2 weeks.

5. Growth-restricted fetuses can be monitored through umbilical artery Doppler velocimetry as a means of avoiding bad perinatal outcomes.


Let's be clear...

- ultrasound (US) technology revolutionized obstetrical and midwifery care

- useful for assessing fetal viability and growth, confirmation of pregnancy dating, measuring blood flow through various veins and arteries of fetus and mom, evaluating fetal anatomy and amniotic fluid, and locating the placenta

- but any intervention, especially one that relies on blasting radiowaves at rapidly dividing cells and tissues has the potential to be harmful

- this shouldn't be a surprise; it's reality

- like any piece of technology, we must not become over-reliant on this technology and coerce our patients into agreeing to its use because the risks are largely unknown


What is US?

- a transducer containing vibrating piezoelectric crystals emits radiowaves at a very high frequency (usually 5-10 MHz --> that's 5-10 million cycles per second)

- lower the frequency, the deeper the penetrance of the waves (worse resolution)

- these waves pass through everything in their path and deflect a portion of their energy back to the transducer

- the proportion of the waves reflected back along with the speed at which they return to the transducer prompts varying electric signals from the transducer to the US computer depending on the density and characteristics of the tissues of the embryo/fetus/mom's body

- the transducer contains an array of crystals such that information is being emitted and received at 1000x per second to create an in-line photo or video that the computer refreshes every 20x per second (this permits real-time motion)

- if you're really interested in the physics of ultrasound read this or watch the video below:



First trimester

- first ultrasound is typically performed in the 1st trimester (<14wga)

- if transabdominal views are insufficient, transvaginal or transperineal approach may be considered

- check the uterus for a gestational sac (GS), yolk sac (SC), and/or embryo +/- cardiac activity

- crown rump length is more accurate than mean sac diameter in estimating gestational age of pregnancy

Source: https://sinaiem.org/

- a GS without an YS or embryo could be a pseudo sac, which is commonly seen inside the uterus in association with an ectopic pregnancy

- cardiac activity should be detectible if the CRL is ≥7mm

- look for multifetal gestations and note amnionicity and chorionicity is possible

- check out the surrounding uterine tissue, adnexa, and cervix if possible


Formal anatomic survey

- generally recommended at 18-22 wga

- box 1 clarifies the anatomic checklist

- placenta accreta/increta/percreta can also be more easily diagnosed at this time

- cervical length is easily measured (particularly if surveillance is warranted)

- estimation of pregnancy dating is less accurate in the 2nd trimester compared to 1st

- biparietal diameter/head circumference (level of thalamus and cavum septi pellucidi), femoral diaphysis length, and abdominal circumference (level of the umbilical vein) are used for dating purposes

dolicocephaly or brachycephaly are normal head shape variants in the 2nd trimester)

- head circumference is not affected by head shape, so may be more accurate in some cases


Note: this scan is extra important for any patient who may consider termination based on structural abnormalities notes on ultrasound, particularly if these abnormalities prompt diagnostic workup that reveals genetic disorders


Growth ultrasound

- several calculators are available in which you input BPD, HC, AC, and FL to predict EFW

- fetal growth is dependent on maternal race and ethnicity, so these factors should be taken into consideration

- if fetal growth is normal but patient is at risk for fetal growth restriction (FGR), serial growth ultrasound may be warranted; you can begin as early as the 2nd trimester

- not recommended that you perform these measurements more frequently than q2 weeks

- fetal weight estimation is notoriously in accurate, with up to 20% error in either direction

- if growth is wonky on your measurement, check dating (and get a second opinion before you go all bananas!)

- if dating checks out, and EFW <10%tile (definition of FGR)

- Doppler velocimetry of the umbilical artery can be an important management tool in counseling around risks/benefits, as this technique allows you to objectify uteroplacental insufficiency (amniotic fluid assessment also helps)

- absent or reversed end-diastolic flow within the umbilical artery is associated with higher perinatal mortality

- Dopplers of MCA and ductus venosus have not yet been determined to be helpful

- FGR fetuses should also be referred for antepartum surveillance (e.g. serial NST or BPP)


Source: www.scielo.br

Source: massonshealthcare.com



3D ultrasound

- can give the appearance of a 3D image by collecting multiple ultrasound images from slightly different directions and creating a composite image

- may be helpful in evaluating severity of facial anomalies, neural tube defects, or skeletal malformations (but mostly just an expensive trick)




Is this technology safe?

- well...maybe?

- ultrasound alone may not be super harmful as the radiowaves are emitted as pulses

- Doppler imaging, on the other, consists of continuous waves, as this is required to visualize very high speed movement (such as that of a very early fetal heart)

- continuous wave emission increases the vibratory rate of the atoms comprising any matter, and this generates heat

- heat can be disruptive to genetic material as well as formed tissues

- so while in theory US can be harmful to pregnancy, human studies have not found any link between use of US and risk of birth defects

- the kicker is that the majority of this research looked at earlier ultrasound technologies that utilized lower acoustic output

- studies have shown that these higher outputs may be harmful to developing fetuses in mammals and human cells in-vitro


"A majority of epidemiologic studies tends to support the safety of diagnostic ultrasound use during pregnancy. However, there have been some reports that there may be a relation between prenatal ultrasound exposure and adverse outcome. Some of the reported effects include growth restriction, delayed speech, dyslexia, and non-right-handedness associated with ultrasound exposure." - Marinac-Dabic et al, Epidemiology, 2002

- in summary, we can't take any technology for granted, and this one may be harmful if a) continuous wave form (Doppler) is used or b) multiple pulsed (non-Doppler) exams are performed, or c) if either method is used and the transducer is focused on embryonic/fetal tissue for too long

- these same concerns are present with the handheld Doppler devices (same technology)

- Chris Kresser did a nice piece on this if you would like more reading (unfortunately not all of his citations are linked)





What's the bare minimum that I suggest for ultrasound in pregnancy?

- For my wife, I will recommend an early ultrasound to confirm dating then a formal anatomic survey, the latter being the single most important

- nuchal translucency ultrasound prior to 14 wga would be a 3rd recommended scan if there was any suspicion for fetal aneuploidies or other genetic disorders based on the 1st ultrasound or serum screening

- then again, if you wouldn't act on the findings, what's the point?

- all scans should be offered to all women (along with risks and benefits) so that they can make an informed decision


When is ultrasound used to change pregnancy dating?

- many patients can't recall the first day of their last bleed (LMP)

- in this case, 1st trimester ultrasound would be used to date the pregnancy

- if the date based on reported LMP differs from the gestational age (GA) estimation by 1st trimester ultrasound by >5-7 days, then the US dating trumps the dating

- in an ideal world, the prediction of due date by LMP will be a day or so within the due date predicted by US, and you know pretty darn accurately which day to circle as your "due date"

- 2nd/3rd trimester dating has slightly different parameters (see Table 1)

- remember that US is notoriously inaccurate in the 3rd trimester for growth/dating purposes, which is why a 21-day margin is provided (don't reassign dating willy nilly later in pregnancy!)


Note: Remember that very few women actually give birth on their due date. My wife and I use the term "guess date" for this very reason...




Single DVP is the way to go for evaluating amniotic fluid volume

- two methods: deepest vertical pocket (DVP) and amniotic fluid assessment (AFI)

- in both cases, make sure your transducer is at a perpendicular angle to the floor

- AFI: divide the uterine cavity into four quadrants, measure the deepest fluid pocket in each quadrant, then add them up

- DVP: measure the single DVP anywhere around the baby



- use of DVP carries a lower false positive rate than AFI; therefore, less unnecessary interventions like induction, c-section, low BPP, etc.

- quit it with your AFI nonsense, ya'll

- normal DVP: 2-8 cm

- normal AFI: 5-25 cm


How is ultrasound incorporated into screening for fetal abnormalities?

- increased nuchal translucency is associated with a broad range of fetal chromosomal, genetic, and structural abnormalities

- normal range of this measurement is dependent on gestational age

- 70% detection rate for trisomy 21 (Down's Syndrome) in high-risk populations

- of fetuses with increased nuchal translucency, 1/3 will have chromosomal defects, half of which will be T21 (most common)

- highest detection rate for aneuploidies: serum biomarkers + gross anomalies like absent nasal bone (also visualized on 1st trimester US), cardiac anomalies, and thickened nuchal translucency/fold

- echogenic bowel, choroid plexus cyst, echogenic intracardiac focus, short femur or humerus, or dilated renal pelvis are considered soft markers --> only meaningful in concert with other significant findings (e.g. abnormal serum screening)

- among high-risk populations, 50-75% detection rate in 2nd trimester (5-15% false positive rate)


Note: Probably a good idea to review PB#226 "Screening for Fetal Chromosomal Abnormalities", which I covered in Episode #54


Prenatal ultrasound can also be used to diagnosis and monitor fetal anemia

- classic story: Rh negative mom gives birth to an Rh positive fetus and develops antibodies to Rh factor during birth. In the next pregnancy, her fetus is Rh positive, and these antibodies attack the fetal blood cells ("alloimmunization")

- in alloimmunization, monitoring for severe anemia can provide insight when intervention might be needed to prevent fetal hydrops (high fatality rate!)

- Doppler velocimetry of the fetal middle cerebral artery can be monitoring serially: if peak systolic velocity goes beyond 1.5 multiples of the median for a particular GA, this is a decent predictor for fetal anemia in the 2nd trimester or early 3rd trimester (75% sensitive)


Final thoughts...on twins

- monochorionic twins have higher risks for poor outcomes (including fetal demise and congenital anomalies), so determining chorionicity early in pregnancy is essential

- keep an eye out for twin to twin transfusion syndrome, which will likely appear as one twin getting too many resource, the other getting too few

- serial surveillance scans are generally recommended to begin at 16 wga (q2 weeks thereafter)



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