Authors evaluated the occurrence of cell free haemoglobin in
the foetal circulation of severe early-onset foetal growth pregnancies, and
measured the autologous heme-handling capacity, to determine whether they are
linked to foetal vascular biometry, indicative of survival risk. Foetal growth
restriction (FGR) represents a failure of a foetus to meet its genetically
determined growth potential. Being born small is associated with stillbirth and
a host of vascular, neurodevelopmental and endocrine conditions in later life,
including, hypertension, diabetes, stroke, coronary artery disease, renal injury
and cognitive impairment. Increased extracellular haemoglobin in plasma has
been implicated in numerous diseases, and vascular dysregulation conditions.
Pertinently, high free haemoglobin overcomes the sequestering capacity of plasma
defences, with deleterious sequalae affecting vascular health.

There is excess free fHbF in FGR neonatal cord blood at late
preterm or term gestations. Authors have also demonstrated that fHbF increases
fetoplacental vascular resistance by sequestering nitric oxide (NO), promoting
matrix damage to the placental stroma, compromising placenta barrier function.
They also reported compromised effects of fHbF on placental endothelial
junctions in the pro-inflammatory and pro-angiogenic effects. The circulating
scavengers, haptoglobin, hemopexin and alpha-1-microglobin (A1M) protect
against fHbF. These endogenous substances are pleiotropic, but within the
context of this study, bind fHbF to protect against the sequestration of
endothelial-derived nitric oxide and prevent fHbF from binding to the
inflammation-evoking TLR4 receptor which is strongly expressed in the placenta,
which activates to elicit expression of numerous inflammatory cytokines
placental in a primary trophoblast cell line. Normally, small amounts of fHbF
are efficiently handled by these systems with limited impact on vascular
function. However, in haemolytic conditions, where there is increased
erythrocyte turnover, including megaloblastoma, excess free Hb occurs, either
oversaturating existing defences, or leading to their compensatory upregulation.

Authors hypothesised that fHbF is elevated in cases of
early-onset FGR, with reduced heme scavenging, and is associated with clinical
features of foetal blood flow redistribution. Previous observations in PE have
demonstrated sexual dimorphism in response to biological challenges, which they
further explored in early-onset FGR.

It was a prospective study severe early-onset foetal growth
restriction pregnancies with close clinical management (estimated foetal weight
(EFW)< 3rd centile and<600 g at 20–26+6weeks; N=20). Temporal foetal
vascular obstetric biometry was recorded. Cord blood fHbF and key
heme-scavenger defences were measured and compared with normal term births
(N=26) and births with late-onset FGR (N=12).

fHbF was elevated in early-onset FGR compared with normal
pregnancy: 0.437(0.337/0.753) mg/mL; and 0.098 (0.045/0.264) mg/mL,
respectively (p<0.0001); whilst hemopexin was downregulated in early-
(p<0.001) and late-onset FGR (p<0.0001), compared to normal pregnancy:
36(14/81) μg/mL, 25(19/40) μg/mL, and 155(132/219) μg/mL, respectively; median
(interquartile ranges).

Early-onset FGR male foetuses had higher HbF compared with
the normal males: 0.710(0.433/0.857) mg/ mL; (p<0.001); 0.099(0.043/0.246)
mg/mL, respectively; median (interquartile ranges).

In early-onset FGR, ratios of mid-cerebral artery and
umbilical artery pulsatility indices correlated positively with heme-scavenger
levels (hemopexin and a heme-handling composite measure: p<0.05, r=0.672;
and p<0.01, r=0.620; respectively), indicating lower levels are associated
with cerebral vascular redistribution.

These heme handling measures also positively correlated with
gestational age at delivery (r=0.713 and r=0.642, respectively, p<0.01,
both) and birthweight (r=0.742, p<0.001; and r=0.523, p<0.05;
respectively).

Fetal cord plasma had significantly higher levels of fHbF in
the EVERREST cohort compared to normal pregnancy delivered at term. Authors
reported significant cord plasma reductionsn hemopexin in both FGR groups
compared to the normal group; being 4–6 times lower in the early- and
late-onset groups, respectively, with an apparent inverse relationship to the
heme load.

To give further strength to the calculation of heme handling,
they derived a composite calculation for each sample. Using this measure,
authors found that earlyonset FGR foetal plasma had a lower score than normal
term samples (p0.0001), more significant than for late-onset FGR(p<0.05). Overall,
they did not show any difference between absolute A1M levels in controls and
FGR groups, though the inclusion of A1M in the aforementioned composite measure
strengthened the separation between early- and late FGR cases.

The haptoglobin heme-defence, was below the level of detection
in all groups using a manual ELISA method (AbCam, ab219048 Human Haptoglobin
SimpleStep ELISA Kit) and by an automated immunoassay analyser method. Two
significant sexual dimorphism findings related only to males: (i) the foetal
plasma fHbF level was higher in early FGR pregnancies than occurred normally at
term; and (ii) matched by an equally significant reduction in hemopexin.

Authors have demonstrated that fHbF is overrepresented in
the severest cases of early onset FGR, with a consequential overwhelming of
heme defences, a scenario linked to poor foetal vascular performance, severe
foetoplacental insufficiency, and preterm delivery. In the early-onset FGR
group, sex-specific variations in response to fHbF and heme appear to exist
with female foetuses displaying higher MCA PI values and a greater proportion
of female foetuses showing MCA/UA ratios in excess of 1, a clinical threshold
below which is associated with increased stillbirth risk. Among this group they
also observed a tendency for female foetuses to have higher hemopexin levels
and more active heme handling profiles

Source: Adam Brook, Georgia Baynes,
Jonathan Scargill; BJOG: An International Journal of Obstetrics &
Gynaecology, 2025; 0:1–11 https://doi.org/10.1111/1471-0528.18104