Fetal hypoxia can arise due to a number of different causes, from infection of the placenta to occlusion of the umbilical cord. The subsequent decrease in oxygen supply to the developing fetus can lead to serious health complications, permanent disability and stillbirth. Current prenatal tests to detect fetal hypoxia, in order to inform the timing of delivery, are unable to determine the severity of the condition. In a recent study in BMC Medicine, Stephen Tong, Clare Whitehead and Susan Walker from Mercy Hospital for Women and the University of Melbourne, Australia, and colleagues, present a new non-invasive maternal blood test to detect and determine the extent of fetal hypoxia. Here, Tong, Whitehead and Walker discuss how their maternal blood test for fetal mRNAs could benefit clinical practice and neonatal outcomes.
What is fetal hypoxia and what are its consequences?
Fetal hypoxia is the dangerous situation of low oxygen levels. Tissues such as the brain – tissues with high energy demands – are particularly sensitive and at risk of permanent damage if oxygen needs aren’t met. Thus, severe hypoxia can result in major disability. Ultimately however, unrelenting hypoxia leads to stillbirth.
There are two clinically important situations of fetal hypoxia. Acute hypoxia can arise over hours during labor, and occurs because each uterine contraction slows maternal blood flow to the placenta.
Placental insufficiency can give rise to chronic hypoxia, where the diseased placenta fails to supply adequate oxygen. If unsuspected, a stillbirth can occur. Of three million annual stillbirths occurring globally, hypoxia may be potentially responsible for half.
How is fetal hypoxia currently assessed?
The underlying principle of all our current tests of fetal hypoxia is that they set out to identify specific fetal physiological responses to hypoxia. For instance, the cardiotocograph is used in labor to detect fetal heart rate patterns associated with the presence of significant acute hypoxia.
Faced with chronic hypoxia, fetuses will grow less (thus, fetal growth restriction flags the presence of chronic hypoxia), move less, produce less amniotic fluid and divert blood flow to the brain. With more critical levels of hypoxia, flow of blood during the diastolic phase of the cardiac cycle can decrease, cease, or even flow backwards. Current tests to identify chronic fetal hypoxia are ultrasound based and seek to capture the presence of these physiological adaptions to low oxygenation.
While these tests have helped improve outcomes, they are not perfect. For example, the cardiotocograph is notorious for overcalling the presence of hypoxia, where clinicians deliver by caesarean section, only to find normal fetal oxygenation levels. Not infrequently, the ultrasound tests to identify chronic hypoxia provide conflicting results (e.g. decreased fetal movements, low amniotic fluid levels but normal fetal blood flow in diastole). Such conflicting findings make it difficult to judge how hypoxic the fetus truly is, and whether immediate delivery is actually required.
What are the clinical implications of having a non-invasive test for fetal hypoxia?
In clinical practice, once the baby is delivered, a sample of blood is taken from the placenta and pH (or lactate levels) is measured. With true fetal hypoxia, the pH will decrease and blood lactate levels will increase. Thus, this readout – which can only be obtained after birth – provides a ‘gold standard’ to reflect the degree of fetal hypoxia during its last moments in utero.
We set out to develop a non-invasive maternal blood test to provide an estimate of fetal blood pH levels in utero. If successful, it could improve on current tests, all of which are relatively insensitive at determining the degree of fetal hypoxia (none appear to be able to provide a reliable estimate of fetal blood pH).
The clinical implications of such a test are that clinicians could better time delivery of fetuses, reducing neonatal morbidity and mortality.
What are hypoxia-induced mRNAs and how could they be used in a test for fetal hypoxia?
With significant cellular hypoxia, hundreds of genes are predictably upregulated, which we call ‘hypoxic-induced mRNAs’.
Amazingly, it was discovered over the past decade that mRNAs of placental origin leak into the maternal circulation where they can be sampled and quantified. We therefore hypothesized when the fetus is hypoxic, it will upregulate and release hypoxia-induced mRNAs into the maternal blood. Furthermore, we hypothesized the relative abundance of these mRNAs would correlate with the degree of hypoxia. If correct, then measuring hypoxia-induced mRNAs could form the basis of a non-invasive clinical blood test of fetal hypoxia.
In our study, we examined acute hypoxia by serially sampling blood from women undergoing labor. We also investigated chronic hypoxia, recruiting women with pregnancies complicated by severe preterm fetal growth restriction. We found hypoxia-induced mRNAs abundance in maternal blood taken close to delivery indeed correlated with the hypoxic status of the fetus during its last moments in utero (determined by measuring fetal blood pH, or lactate levels from the placenta at birth).
Thus, our data provides proof of principle evidence that measuring hypoxia-induced mRNAs abundance in maternal blood could be used to determine the degree of fetal hypoxia in utero.
How were you able to determine that the mRNAs you detected were from the fetus and not the mother?
We did not definitely prove the mRNAs we were measuring in the maternal blood were from the fetus or placenta. However, as discussed in our paper, we feel we have presented strong circumstantial evidence to suggest they were of placental origin.
Ultimately, if hypoxia induced mRNA were validated to accurately reflect fetal hypoxia status in utero, it would not be absolutely essential to establish their origin, although we think a fetal/placental source is most likely.
It is technically possible to prove that the hypoxia-induced mRNA are of placental origin using next generation sequencing technologies. mRNA sequence information can be compared with the maternal and fetal genomes. Such studies are on the cards.
In what situations could this test be used?
We believe there may be a variety of clinical situations where this test to determine fetal hypoxia levels could be useful. However, we think the greatest impact could be in monitoring fetuses that are growth restricted at very preterm gestations.
With preterm fetal growth restriction, the risks of stillbirth are high and clinicians face the tricky situation of having to time delivery. The clinician is required to balance the probability of stillbirth, or permanent disability (caused by leaving the baby too long in an environment of severe chronic hypoxia) if the pregnancy is left to continue versus the risk of causing ‘iatrogenic’ prematurity if the preterm fetus is delivered unnecessarily early (severe prematurity has, itself, serious complications including cerebral palsy).
A test that is able to more precisely estimate fetal blood pH in utero could be used to help the clinician make more informed decisions regarding delivery, improving outcomes.
What’s next for your research?
Excitingly, we have implemented a large observational multicentre study (seven referral centres in Australia and New Zealand) to verify our findings and develop a test with clinical care in mind. Called the ‘FOX study’ (Fetal Oxygenation study), we hope to recruit 180 participants with severe preterm growth restriction. We hope to confirm the inverse correlation between hypoxic–induced mRNAs abundance and fetal blood pH determined at delivery, but with greatly expanded numbers.
The vision is that we will develop a tool where clinicians can measure hypoxic-induced mRNAs in the maternal blood, look up a ‘reference’ chart that this study will generate, and obtain an estimate of the fetal blood pH. Hypothetically, an estimated pH reading of 7.3 would suggest the fetus could be safely left to gain gestation, whereas, a pH of 7.05 would mandate delivery.
If the FOX study validates our hypothesis, we believe this test has exciting potential to improve fetal outcomes and perhaps decrease the burden of major disabilities and stillbirth.
Questions from Joanna Denyer, Senior Assistant Editor for BMC Medicine and Stephanie Harriman, Deputy Medical Editor for BioMed Central.