Significant fetal acidemia at birth is strongly associated with perinatal death and adverse perinatal complications, including permanent neurological disability  and cerebral palsy . In a study of 60 preterm fetuses delivered at ≤28 weeks gestation, an umbilical artery blood pH of ≤7.15 was strongly associated with severe adverse neurological outcomes (sensitivity 30% at 98% specificity) compared with higher pH levels . In another study of 604 neonates delivered at ≤33 weeks gestation, an umbilical cord pH of ≤7.20 was associated with a 4.2 likelihood ratio of fetal death . Therefore, a non-invasive test that can estimate fetal acidemic status could help clinicians’ better time delivery. While current non-invasive antenatal tests can identify fetuses at higher risk of being acidemic, none have been validated to accurately estimate the degree of fetal acidemia in utero.
Here we have presented evidence to suggest quantifying hypoxia-induced mRNA in the maternal circulation may be a novel approach to determining in-utero fetal hypoxic status. Hypoxia-induced transcripts in the maternal circulation appear tightly correlated with expression in human gestational tissues, and they dynamically change with acute alterations in presumed fetal hypoxic status. Furthermore, we generated a hypoxia gene expression score that sums the relative abundance of mRNA in the maternal circulation that code four hypoxia-induced genes. This score appeared to be highly correlated with acute (labor cohort) and chronic (FGR cohort) fetal hypoxia.
While the measurement of free mRNA in the maternal circulation has been studied previously, we believe our study represents a significant conceptual advance. Previous studies have proposed the use of free mRNA as a ‘static’ tool, where levels are measured once in order to either diagnose [23, 24] or predict pregnancy complications [25–27]. Here we propose serial measurements to observe dynamic changes within the same patient, monitoring hypoxic status over time and delivering when significant acidemia is predicted.
The cardiotocograph is the mainstay of monitoring to identify hypoxia during labor. While it performs well in identifying the presence of fetal hypoxia (85% sensitivity) , its specificity is notoriously poor because heart rate decelerations, including late decelerations, can either be caused by hypoxia or be induced by mechanical reflex autonomic responses unrelated to hypoxia. As a result, use of the cardiotocograph results in unnecessary interventions . Ours may be the first ‘theoretical’ non-invasive test for women in labor that can determine the degree of in utero fetal acidemia. The speed of current PCR technologies means such a test is not feasible as a clinical tool to make decisions during labor but improvements in nucleic acid detection technologies might make such a test possible in the future.
We have also presented evidence suggesting hypoxia-induced mRNA in the maternal circulation correlates with acidemic status of FGR fetuses’ in utero. It is conceivable that day-to-day clinical decisions regarding timing of an FGR fetus can await the results of a PCR result performed using machines available today. Therefore, our test may have a role in situations where current tests of fetal well-being are equivocal and the clinician is left unsure whether the fetus should be delivered. This occurs quite frequently. A prospective study examining a preterm FGR cohort found biophysical profile results were discordant with the umbilical artery Doppler findings in 55% of cases . Thus, a test that can provide a reliable estimate of in utero fetal blood pH levels in such situations may help clinicians decide whether immediate delivery is warranted.
A limitation of our study is that we have not decisively proven the hypoxia-induced mRNA we are measuring in the maternal blood originates from the fetoplacental unit. This may be possible with the use of next-generation sequencing technologies where sequence information could be used to identify the origin of mRNA transcripts (maternal or fetal). However, we have presented strong circumstantial evidence to suggest the hypoxia induced mRNA are of fetoplacental origin: 1) they increase with situations of likely severe acute and chronic fetal hypoxia, 2) they correlate with an increase of hypoxic mRNA transcripts in gestational tissues, and 3) their relative abundance displays a highly significant and tight correlation with fetal acidemic status at birth. Ultimately, if hypoxia-induced transcripts in maternal blood were validated to reflect fetal acidemic status, it would not be absolutely essential to establish their origin, although a fetoplacental source seems the most likely.
To translate our potential test to the monitoring of fetuses with severe FGR, our test requires validation with a study of larger numbers. Such a validation study could also help determine whether clinical factors, such as smoking and maternal obesity, alter hypoxia induced mRNA levels in maternal blood. We are currently undertaking such a large prospective validation study.
Furthermore, in this proof of concept study, we summed the relative expression of mRNA in maternal blood that codes Hif1α, Hif2α, LdhA and Adm to generate a gene hypoxia score. These genes were chosen on the basis of their biology; the former three have central roles in the hypoxic response , and Adm is both hypoxic regulated  and very highly expressed in placenta. Future studies should bioinformatically screen other hypoxia-induced genes to develop the most accurate test to determine degree of in utero fetal acidemia. Finally, it may be more optimal to develop a clinical test that expresses mRNA abundance by copy number rather than relative expression.
In conclusion, we have presented evidence to show measuring circulating hypoxia-induced transcripts in maternal blood may be a promising approach to clinically assess fetal hypoxic status in utero. It may be useful to help clinicians’ time delivery, especially in cases of severe preterm FGR, potentially improving perinatal outcomes and decreasing rates of stillbirth.