Antenatal Testing Modalities
/Modalities of Antepartum Fetal Testing
Extra Reading:
ACOG PB 229 (Antepartum Fetal Surveillance)
CO 828 (Indications for Antepartum Fetal Surveillance)
What we won’t do:
Talk about frequency / timing / specific indications for testing
CO 828 provides a great overview of this as a reference, and we’ll obviously mention our own practice for testing in subject-specific episodes!
Various institutions may have their own policies and procedures that you should be aware of.
Rationale / Techniques of Antenatal Testing
Testing techniques are meant to detect changes in fetal status, particularly in response to hypoxemia and acidemia.
Hypoxemia and resulting metabolic acidemia redirect fetal blood flow; for instance, decreasing renal perfusion and resulting on oligohydramnios.
Other physiologic changes such as decreased heart rate variability and decreased fetal movement or poor muscle tone can be appreciated.
Progressive metabolic acidosis In these situations can result in stillbirth / fetal death – the primary outcome we are looking to avoid by employing this testing.
Antepartum surveillance is generally indicated for any condition which raises the risk of stillbirth.
In CO 828, the “cutoff” for included indications is defined as 0.8 / 1000 deliveries for a given condition, which is about twice the odds of stillbirth at term in otherwise low risk pregnancies
This is also the false-negative rate of a BPP, providing further justification for this cutoff.
Importantly, any fetus with neurologic depression/anomalies may still be at increased risk for stillbirth, but antenatal testing may not be as equipped to detect risk.
Similiarly, the utility and predictive value are less at earlier gestational ages.
Maternal kick counts
A complaint of decreased in fetal movement has been demonstrated as a risk for stillbirth, which is why we ask folks to come to be assessed for it!
How effective are routine kick counts?
A meta-analysis of five RCTs and over 450,000 fetuses found no differences in stillbirth rate between groups undergoing routine kick counts, and those who did not.
However, stillbirth rate in the trials overall was low – 0.54% in the kick count group, and 0.59% in the control group, with the confidence interval for the relative risk ranging from 0.85 – 1.00 – so while crossing 1 and thus being statistically non-significant, may lay more towards benefit with additional study.
Those in the kick count group did experience slightly higher rates of preterm delivery, labor induction, and cesarean delivery – perhaps some harm from this?
Likely, more studies are necessary to help make better determinations regarding the utility of kick counts.
How should we tell people to do kick counts?
The practice bulletin mentions one trial instructing patients to lie on their side and count 10 movements over a 2 hour period as reassuring – we trained with this as the recommended way!
The bulletin also mentions another to count movements for 1 hour three times per week, and reassuring was considered to be if the movement number “equals or exceeds the previously established baseline count” – seems a bit more complicated!
There definitely isn’t robust evidence to do routine fetal movement assessments, or to use that as a testing technique – but again, definitely have your patients present to care for any sense of decreased movement!
Non-Stress Test (NST)
The NST is the classic method for antenatal testing. It’s based on the notion that if the fetus is not acidotic, the heart rate variability will be appropriate, and the heart rate will accelerate with fetal movement.
The heart rate should be monitored for at least 20 minutes, but may require longer periods due to fetal sleep cycles.
Vibroacoustic stimulation (VAS) is permissible to obtain a stimulated acceleration – it won’t falsely reassure you abut acid-base status, and can decrease the amount of time you’re waiting through a sleep cycle!
Per the PB, stimulus should be applied for 1-3 seconds and may be repeated up to three times for a valid NST.
Test results are conveyed as reactive or non-reactive.
Even though this is the same tech as continuous monitoring in labor, we don’t use the same terminology! So don’t call an NST “category 1” – the only results are reactive or non-reactive.
Reactive means that there should be two or more accelerations in a 20-minute period.
Accelerations should be 10 beats elevation over 10 seconds (10x10) at less than 32 weeks, and 15 x 15 at greater than 32 weeks.
Non-reactive means that over 40 minutes of monitoring, the acceleration criteria is not met.
So can you have decels in a “reactive” NST? – yes! Remember – “category” system in labor is different than NST description!
Variable decelerations that are non-repetitive and brief (< 30 sec) are not associated with fetal compromise or need for obstetric intervention.
If repetitive (>3 in 20 mins), there is increased risk for cesarean delivery for non-reassuring monitoring, and so should be considered for additional testing.
Decelerations lasting for over 1 minute during an NST is associated with a high risk of cesarean as well as stillbirth – so additional testing or delivery should be considered if present.
One common question – when should an NST be reactive?
This is hard to answer – studies have looked at this using the original 15x15 criteria:
At 24-28 weeks, up to 50% of NSTs are non-reactive
At 28-32 weeks, up to 15% of NSTs are non-reactive
For this reason, the 10 x 10 criteria was proposed and deemed sufficient, but certainly the rate of non-reactive tracings are much higher at earlier gestational ages, and so some institutions adjust monitoring protocols accordingly prior to 28-32 weeks.
Contraction Stress Test (CST)
A step-up from the NST – now, we look to interpret the fetal heart rate response to provoked uterine contractions.
Since contractions cause some transient fetal deoxygenation, a fetus that is compromised already will demonstrate inability to compensate physiologically – and thus show decelerations.
There’s a lot of vocabulary regarding the CST, so let’s start with how the test is run:
An adequate CST requires three contractions persisting for at least 40 seconds each in a 10 minute period. If the CST is inadequate, contractions can be stimulated with IV oxytocin or nipple stimulation.
In this testing, nipple stimulation actually is very useful, and can achieve adequate testing in half the time required versus IV oxytocin!
Once the test is adequate, it should be monitored similarly to an NST over 20 minutes. The test results can be one of five options:
Negative: No late or significant variable decelerations elicited. A “negative” CST means the fetus is not compromised.
Positive: late decelerations are present after 50% or more of contractions. This is also a possible interpretation even with an inadequate CST. Thus, a “positive CST” suggests compromise of the fetus.
Equivocal-suspicious: Intermittent late decelerations or significant variable decelerations are present.
Equivocal: FHR decelerations in the presence of contractions that are occurring more frequently than every 2 minutes, or lasting longer than 90 seconds. This suggests that tachysystole or contraction strength may be compromising the validity of the test.
Unsatisfactory: synonymous with an inadequate CST – that is, fewer than three contractions in 10 minutes; alternatively, if the tracing is uninterpretable.
CSTs are great and often overlooked tools but should not be performed, generally speaking, in conditions that are contraindications to labor or vaginal delivery – i.e., placenta previa.
Biophysical Profile (BPP) and Modified BPP
The BPP combines an NST with up to four ultrasound criteria, which are:
Amniotic fluid volume: per the practice bulletin, a single deepest vertical pocket of greater than 2 cm is adequate.
Some practices may use the amniotic fluid index instead, or in addition to, a measurement of the deepest vertical pocket. RCTs suggest that DVP is acceptable and may even be preferred.
Fetal movement: three or more discrete body or limb movements.
Fetal tone: one or more episodes of extension of a fetal extremity with return to flexion, or opening and closing of a hand.
Fetal breathing movements: One or more episodes of fetal breathing movements of 30 seconds or more.
All of the ultrasound criteria should be observed in 30 minutes or less.
Each component is scored as a 0 (if criteria not met) or 2 (if criteria met), including the NST.
8-10 / 10: normal
6 / 10: equivocal - generally repeat in 6-12 hours
4 or less / 10: abnormal -- context dependent - deliver or repeat testing pending situation
The modified BPP consists of just the NST in addition to assessment of amniotic fluid volume. If either of these is abnormal, the usual next step is to proceed with the remainder of the BPP.
Oligohydramnios is a separate and significant risk for stillbirth – remember that delivery is recommended for this starting at 36’0 or later! Additional testing should definitely be performed if this is seen earlier than 36 weeks.
If you’re seeing lots of decels – then resuscitate/deliver!
A BPP can’t reassure you about decels, which suggest some sort of acute decompensatory event is occurring.
Umbilical Artery Doppler Velocimetry (UAD)
UAD is a special ultrasound technique that is generally most useful in monitoring fetuses affected by growth restriction.
The umbilical artery blood flow in cardiac diastole is very high in normally-growing fetuses.
In the growth restricted fetus, blood flow in diastole may be reduced, absent, or reversed; and with these abnormalities, perinatal mortality and stillbirth risk are significantly increased.
This is particularly true in the presence of absent or reversed end-diastolic flow.
There is no evidence at present that UAD helps to provide more information about fetal well-being in other situations; so FGR is the only place you’ll routinely see it employed in obstetrics.
Listen to our FGR podcast to learn more!
What is the comparative predictive value/efficacy of these tests?
The evidence for antenatal testing is largely circumstantial; that is, stillbirth risk seems reduced compared to unmonitored pregnancies, which are largely historic cohorts before current technology was available.
We can state with confidence that testing has high negative predictive value.
Normal test results in most cases are highly reassuring, as these tests are associated with low false-negative rates – with a false negative defined as a stillbirth within 1 week of the normal result. Fer each modality:
NST: 1.9 per 1,000; aka, negative predictive value of 99.8%
CST: 0.3 per 1,000: aka, NPV of >99.9%
BPP: 0.8 per 1,000: aka, NPV of >99.9%
Modified BPP: 0.8 per 1,000: aka, NPV of >99.9%
UAD: less studied, but best available evidence suggests an NPV approaching 100% as well.
Again, these tests can help reduce risk of stillbirth with chronic conditions; acute conditions, such as abruption or cord accident, cannot be predicted with these modalities.