The relationship between maternal anaemia and fetal heart rate patterns

At first glance you would think the relationship between maternal haemoglobin levels and fetal heart rate patterns would be fairly straight forward. Haemoglobin carries oxygen. More haemoglobin means better oxygen carrying capacity and that should mean more oxygen delivery to the fetus. Better fetal oxygenation should mean more accelerations and fewer of the fetal heart rate patterns typical of hypoxia – like late or recurrent variable decelerations. Simple, right?

Maybe not…

New research

A research team based in the United States recently examined the relationship between anaemia in the pregnant woman (low haemoglobin concentration) and the fetal heart patterns detected by CTG monitoring during labour. 8,196 women with singleton pregnancies at term, who gave birth between 2010 and 2014, and who had a full blood count on admission in labour, cord blood gases at birth, and continuous intrapartum CTG monitoring during the final hour of labour were included. Maternal anaemia was defined as a haemoglobin level of 11 g/dL or less.

33% of the population were anaemic by these criteria. Women in spontaneous labour and those being induced were included. Oxytocin use was significantly more common in women considered anaemic (71% vs 65%, p <0.01). Epidural use, a history of prior caesarean section, having given birth before, and being younger were all more common in women with anaemia.

Findings

Women considered to be anaemic were less likely to have an abnormal (type II by the NICHD criteria) fetal heart rate pattern in the hour prior to birth (after correcting for induction of labour and hypertensive disorders of pregnancy). While the absolute difference was small (32% vs 34%) it achieved statistical significance. Looking at the individual elements of the CTG recording, women with anaemia had fewer recurrent variable decelerations. There were no differences in reduced variability, recurrent late decelerations, persistent tachycardia, or prolonged decelerations. Total deceleration area was increased in women with haemoglobin levels over 11 g/dL. The strength of these findings increased when women with moderate to severe anaemia (haemoglobin less than 8.9 g/dL) were compared to women with haemoglobin levels over 11 g/dL.

What does this mean?

The physiological relationship between maternal haemoglobin and fetal heart rate patterns (assumed to reflect fetal oxygenation) is clearly more complex than the logic line I set out in the first paragraph. The authors describe findings from other research highlighting physiological adaptations to maternal anaemia and hypoxia:

Placental changes leading to increased density of blood vessels and therefore better oxygen exchange
Larger sized placentas
Thinning of the villous membrane so the distance between maternal and fetal circulations is less
Increased levels of lactic acid favouring the release of oxygen from maternal haemoglobin

From these findings and their own, it seems maternal anaemia might have some advantages for the fetus.

Of course, there are other things to consider here – such as the cause of the anaemia. Iron deficiency causes both anaemia in the mother and reduced myelination of the developing fetal brain, with long term impacts on neurological outcomes (Georgieff, 2020). Care needs to be taken to look at a broad range of consequences of our efforts to screen for, prevent, and treat maternal anaemia in pregnancy. The findings of this research are an important reminder that we shouldn’t take assume what seems logical is actually what happens.

Reference

Beermann, S. E., Watkins, V. Y., Frolova, A. I., Raghuraman, N., & Cahill, A. G. (2023, Apr 20). The relationship between maternal anemia and electronic fetal monitoring patterns. American Journal of Obstetrics & Gynecology, in press. https://doi.org/10.1016/j.ajog.2023.04.018

Georgieff, M. K. (2020, Oct). Iron deficiency in pregnancy. American Journal of Obstetrics & Gynecology, 223(4), 516-524. https://doi.org/10.1016/j.ajog.2020.03.006