Relation between maternal haemoglobin concentration and birth weight in different ethnic groups.
ABSTRACT To assess the relation of the lowest haemoglobin concentration in pregnancy with birth weight and the rates of low birth weight and preterm delivery in different ethnic groups.
Retrospective analysis of 153,602 pregnancies with ethnic group and birth weight recorded on a regional pregnancy database during 1988-91. The haemoglobin measurement used was the lowest recorded during pregnancy.
North West Thames region.
115,262 white women, 22,206 Indo-Pakistanis, 4570 Afro-Caribbeans, 2642 mediterraneans, 3905 black Africans, 2351 orientals, and 2666 others.
Birth weight and rates of low birth weight (< 2500 g) and preterm delivery (< 37 completed weeks).
Maximum mean birth weight in white women was achieved with a lowest haemoglobin concentration in pregnancy of 85-95 g/l; the lowest incidence of low birth weight and preterm labour occurred with a lowest haemoglobin of 95-105 g/l. A similar pattern occurred in all ethnic groups.
The magnitude of the fall in haemoglobin concentration in pregnancy is related to birth weight; failure of the haemoglobin concentration to fall below 105 g/l indicates an increased risk of low birth weight and preterm delivery. This phenomenon is seen in all ethnic groups. Some ethnic groups have higher rates of low birth weight and preterm delivery than white women, and they also have higher rates of low haemoglobin concentrations. This increased rate of "anaemia," however, does not account for their higher rates of low birth weight, which occurs at all haemoglobin concentrations.
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ABSTRACT: The association between anemia during pregnancy and spontaneous preterm birth was studied with a two-stage case-control design in a large, multiethnic cohort. Results of all hematologic measurements were abstracted from the prenatal and delivery records of 1706 of the 26,901 women in the cohort. Among women delivered of infants at term, mean hematocrit value was low during the early phase of the second trimester, stable until near term, then reached a maximum at 40 weeks' gestation. The mean hematocrit value of black women was consistently lower than that of Asian, Mexican, and white women. Anemia (hematocrit value less than the tenth percentile for ethnic group and duration of pregnancy) at any time during the second trimester was positively associated with subsequent spontaneous preterm birth (odds ratio, 1.9; 95% confidence interval, 1.3 to 2.8). Compared with white women, the odds ratios for preterm birth were 2.0 (95% confidence interval, 1.6 to 2.4) for black, 1.2 (95% confidence interval, 0.9 to 1.6) for Asian, and 1.2 (95% confidence interval, 1.0 to 1.5) for Mexican women. Adjustment for second-trimester anemia had minimal influence on the odds ratios. We conclude that anemia during the second trimester was associated with preterm birth. However, it does not account for the large ethnic differences in preterm birth.American Journal of Obstetrics and Gynecology 02/1991; 164(1 Pt 1):59-63. · 3.88 Impact Factor
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ABSTRACT: Maternal Hb levels during the third trimester were studied in relation to certain maternal and fetal parameters in 877 apparently normal pregnancies. Low Hb levels at term were closely associated with increased frequency of newborns in the heavy weight-for-date group. Conversely, high maternal Hb levels were closely associated with an increased frequency of newborns in the light weight-for-date group. The maternal Hb levels both in the early third trimester and at term were significantly higher in mothers of small-for-date newborns than in those with newborns of normal weight. In both groups the maternal Hb levels increased significantly during the third trimester of pregnancy. High maternal Hb levels both early and late in the third trimester of pregnancy should be a matter of concern rather than of reassurance.Acta Obstetricia Et Gynecologica Scandinavica 02/1984; 63(3):245-8. · 1.85 Impact Factor
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ABSTRACT: Red cell mass, derived from plasma volume and haematocrit, was calculated serially in 45 healthy women during pregnancy and 4 to 6 months after delivery. Compared to the non-pregnant, postpartum value there was a reduction of approximately 100 ml at 12 weeks gestation. An increase in red cell mass occurred thereafter: at 36 weeks gestation values 180 ml greater than the non-pregnant were found. Iron and folic acid supplements almost doubled this increment to 349 ml of red cells.British Journal of Obstetrics and Gynaecology 06/1979; 86(5):364-70.
Relation between maternalhaemoglobin concentration and birth
weight in different ethnic groups
Philip Steer, M Ash Alam, Jane Wadsworth, Anne Welch
Objective-To assess the relation of the lowest
haemoglobin concentration in pregnancy with birth
weight and the rates oflow birth weight and preterm
delivery in different ethnic groups.
Design-Retrospective analysis of 153 602 preg-
nancies with ethnic group and birth weight recorded
on a regional pregnancy database during 1988-91.
The haemoglobin measurement used was the lowest
Setting-North WestThames region.
Pakistanis, 4570 Afro-Caribbeans, 2642 mediter-
raneans, 3905 black Africans, 2351 orientals, and
Main outcome measures-Birth weight and rates
oflow birth weight (<2500 g) and preterm delivery
(< 37 completed weeks).
Results-Maximum mean birth weight in white
women was achieved with a lowest haemoglobin
concentration in pregnancy of 85-95 g/l; the lowest
incidence of low birth weight and preterm labour
occurred with a lowest haemoglobin of 95-105 g/l. A
similar pattern occurred in all ethnic groups.
Conclusions-The magnitude ofthe fall in haemo-
globin concentration in pregnancy is related to birth
weight; failure of the haemoglobin concentration to
fall below 105 g/l indicates an increased risk of
low birth weight and preterm delivery. This pheno-
menon is seen in all ethnic groups. Some ethnic
groups have higher rates of low birth weight and
preterm delivery than white women, and they also
have higher rates of low haemoglobin concentra-
tions. This increased rate of "anaemia," however,
does not account for their higher rates of low
birth weight, which occurs at
During normal pregnancy in women not given
supplementary iron the haemoglobin concentration of
the maternal blood falls from a non-pregnant average
of about 133 g/l to an average of about 110 g/l at 36
weeks.' The fall is steepest up to 20 weeks' gestation;
the concentration remains fairly constant up to 30
weeks and then rises slightly thereafter.23 Thus, any
estimation of haemoglobin concentration taken after
20 weeks' gestation will be reasonably representative of
the fall induced by pregnancy. Whether this represents
anaemia or not remains a controversial issue. The
World Health Organisation in its nutritional report of
1972 suggested that anaemia was likely to be present in
pregnant women when the haemoglobin fell below
110 g/l4; this will therefore include about half of all
unsupplemented women. A recent paper reporting a
study of 8684 pregnant women in Oxford (one of the
most affluent areas of the United Kingdom) suggested
that 47% were iron deficient and that this had adverse
effects on the ratio of the placental weight to birth
weight, which could lead to hypertension in later life.5
Supplementation of pregnant women with iron
and folic acid reduces the incidence of haemoglobin
< 110 g/l to under 50/0.1 As a result, routine prophylaxis
is commonly advocated.67 In contrast, a recent review
of 20 randomised controlled
"routine supplementation has no detectable effect on
any substantive measures of either maternal or fetal
outcome,[in particular] on preterm delivery, low
stillbirths or neonatal morbidity. The
available data from controlled clinical trials provide no
convincing case for routine as opposed to selective iron
supplementation. This may be a reflection of the low
prevalence of iron deficiency likely in most of the
populations studied. Routine supplementation may be
warranted in populations in which iron deficiency is
Most of the obstetric units in the North West
Thames region have for the years over which our
dataset extends (1988-91) used a common maternity
information system (the St Mary's maternity informa-
tion system, SMMIS) and have thus created a large
database available for examination. We used
examine the relation between the lowest haemoglobin
concentration in pregnancy, birth weight, and preterm
delivery after controlling for ethnic group.
trials reported that
The total dataset comprised 157996 pregnancies
resulting in a registrable birth (24 or more weeks'
gestation and including live and stillbirths). Ethnic
origin and birth weight were available for 153602
(4394 cases with missing data (2-8%); ethnic group
missing in 4393 and birth weight in 86). Of these,
115262 were classified as white (73%), 22206 as
Indo-Pakistani (14%), 4570 as Afro-Caribbean (2-9%),
3905 as black African (2-5%), 2642 as mediterranean
(1-7%), 2351 as oriental (1-5%), and 2666 as other
ethnic group (1 7%). Coding ofethnic group was done
at booking by the midwife after consideration of the
woman's family history and appearance and taking into
account the woman's own views. All women whose
families derived from the Indian subcontinent were
classified as Indo-Pakistani, and all Chinese, Japanese,
and similar races were classified as oriental. Women
who could not be classified into any ofthe main groups
(either because ofan unusual race or mixed parentage)
were classified as other. Data analysis was performed
with SAS (Version 6) run in a Sun minicomputer with
Gestational age was entered as the best estimate by
the midwife after the birth of the baby as part of the
birth notification process, by taking into account the
menstrual history and early ultrasound measurements
when performed. Data on gestational age were missing
for 134 cases (0-08%), commonly in cases already
excluded from analysis because of missing birth
Academic Department of
Medical School, Chelsea
Philip Steer, professor
St Bartholomew's Hospital
Medical College, London
M Ash Alam, medical student
Academic Department of
Public Health, StMary's
Jane Wadsworth, senior
Academic Department of
Anne Welch, research
weight. Low birth weight was defined as <2500 g
and preterm birth as before 37 completed weeks of
Lowest haemoglobin values were entered by the
midwife after the birth of the baby as part of the birth
notification process after inspection of all the measure-
ments made during pregnancy. Standard practice in all
the hospitals entering data was to take blood for
haemoglobin estimation at booking, at the end of the
second trimester (26-28 weeks), and again half way
through the third trimester (34-36 weeks). Data were
missingin 8442 cases (5 3%). For ease ofinterpretation,
haemoglobin values were classified into groups each
spanning 10 g/l, centred around decile values. The
number of cases in white women (109 050) was suffi-
cient to allow creation ofeight groups, from S 85 g/l to
> 145 g/l. The total data were then analysed by using
the SAS procedure "logistic" to create logistic models
of the relations between haemoglobin concentration
and low birth weight and preterm delivery, allowing
for the differences between ethnic groups and other
potentially confounding variables.
Analysis of data for white women showed that mean
birth weight rose as haemoglobin concentration fell
(table I), with the highest mean birth weight of 4483 g
in the 86-95 g/l haemoglobin band, 545 g higher than if
the lowest measurement ofhaemoglobin concentration
was > 145 g/l. Even more striking was the U shaped
distribution of the rate of low birth weight by haemo-
globin group (fig 1), with the incidence of low birth
weight being least at 96-105 g/l. The major factor
TABLE i-Mean (SD) birth weight and incidences oflow birth weight
(<2500g) andpreterm births (<37completedweeks) by haemoglobin
concentrationfor white women in North West Thames region
at birth (%/)
concentration (g/l) pregnancies
birth weight (g)
TABLE II-Birth weight by haemoglobin concentration for 40, 38, and 36 weeks' gestation. Data for white
women in North West Thames region
40 Weeks' gestation
38 Weeks' gestation
36 Weeks' gestation
Mean (SD) birth
Mean (SD) birth
Mean (SD) birth
TABLE III-Incidence ofpreterm birth (<37 completed weeks), incidence of low birth weight (<2500 g),
mean (SD) birth weight, andmean (SD) haemoglobin concentrationfor different ethnicgroups
at < 37 weeks'
Haemoglobin concentration (gil)
FIG 1-Incidence of low birth weight (<2500 g) by haemoglobin
concentration (gil) (dataforwhite women only)
% OS Nd \\%\¢
Haemoglobin concentration (g/l)
FIG 2-Incidence ofpreterm labour (<37 completed weeks) by
haemoglobinconcentration (gll) (datafor white women only)
Haemoglobin concentration (g/l)
FIG 3-Incidence of low birth weight in different ethnic groups by
haemoglobin concentration (gil)
determining birth weight was the incidence of preterm
delivery (fig 2), although the same pattern of relation
between birth weight and haemoglobin concentration
was seen even when we controlled for gestational age
There were differences in the incidence of low
haemoglobin concentration, low birth weight, and
preterm delivery between the ethnic groups (table III),
notably between white women and Africans, Afro-
Caribbeans and Indo-Pakistanis. The logistic model,
however, showed that ethnic origin had no significant
effect on the relation between haemoglobin concentra-
tion and birth weight-that is, the higher rate of low
birth weight in the ethnic minorities was seen at all
haemoglobin concentrations (fig 3). Table IV shows
the results of the logistic model after allowing for the
BMJ VOLUME 310
TABLE IV-Adjusted odds ratios* (95% confidence intervals) for low
birth weight (<2500g) andpreterm birth (<37 completed weeks)
Adjusted odds ratio
(95% confidence interval) (95% confidence interval)
for birth weight
< 2500 g
Adjusted odds ratio
< 37 weeks
1-62 (1-27 to 2 06)
1-04 (0-89 to 1-04)
1-14 (1-06to 1-24)
1-62 (1-50 to 1-62)
2-43 (2-22 to 2-66)
3-37 (2-88 to 3 95)
7-54 (4 95 to 1 1 * 1 1)
1-72 (1-35 to 2-18)
1-21 (1-05 to 1-39)
1 12 (1-04to 1-21)
1-49 (1-38 to 1-61)
2-20 (2-02 to 2-41)
3-04 (2-60 to 3-56)
5-61 (3 71 to 8 49)
*Adjusted for ethnic group, body mass index (weight (kg)/(height (m))2),
parity, maternal age, and smoking.
effects ofethnic origin and other potential confounders.
They show that the incidence of low birth weight was
significantly raised (compared with the incidence at
96-105 g) below 85 g/l and at all concentrations of
haemoglobin above 105 g/l. Exclusion of cases ending
in stillbirth made no significant difference to any ofthe
Our study has shown the striking association ofbirth
weight with haemoglobin concentration. Not only was
there a five to sevenfold increase in preterm delivery
and low birth weight ifthe lowest haemoglobin concen-
tration measured during pregnancy was above 145 g/l
but also the concentrations of haemoglobin associated
with the highest mean birth weight (86-95 g/l) were
much lower than is commonly appreciated. Birth
weight is the single biggest determinant of mortality in
the first year oflife,9 and has therefore a strong claim to
being a good indicator of the efficiency with which a
woman has supported her fetus. By this criterion, a
mid-trimester fall of haemoglobin concentration to
about 100 g/l seems to be optimal. This probably
reflects good expansion of plasma volume.°'0' Failure
of the haemoglobin concentration to fall is known to
be associated with up to a threefold increased risk of
pre-eclampsia,"2 the birth of small for gestational age
infants,'2-'7 and preterm delivery.918 This
apparent even in relation to the haemoglobin concen-
tration at booking, before completion ofthe pregnancy
induced fall.'2 These phenomena are probably closely
interrelated. The mechanism by which expansion of
the plasma volume enhances fetal growth is not known,
but it may be that reduced blood viscosity favours
blood flow in the low velocity flow system of the
maternal intervillous space.
Haemoglobin concentrations <95 g/l seem to be
remarkably harmless. Our study has shown that,
although low haemoglobin concentrations are more
common in some ethnic minorities, notably Africans,
* Haemoglobin concentrations as low as 95 g/l
in pregnancy were associated with optimal fetal
* Failure of the haemoglobin concentration to
fall during pregnancy was associated with a five
to sevenfold increase in the incidence of low
birth weight and preterm birth
* Routine haematinic administration in preg-
nancy is unnecessary on fetal grounds for most
* The mean corpuscular volume may be a
better indicator of maternal iron deficiency, but
this needs prospective study
Afro-Caribbeans, and Indo-Pakistanis, this does not
account for the increase in low birth weight and
preterm labour in these groups, which persists at all
haemoglobin concentrations. It may be that haemo-
globin concentrations, birth weight, and length of
gestation are genetically programmed to be different in
these groups and are therefore physiological, although
their higher rates of clearly adverse outcome such as
perinatal mortality mean such a conclusion should
properly be drawn only after more detailed studies of
Many authors have drawn attention to the fact that
we should be more concerned about high rather than
low concentrations of haemoglobin in pregnancy,9 12-'8
and some have decried the routine use of iron supple-
mentation in pregnancy.'9-21 There
risk that increasing haemoglobin concentration might
actually reduce birth weight and do harm, although the
meta-analysis of the controlled trials does not support
seems simply to be unnecessary.
that genuine anaemia
opposed to a low haemoglobin value) has adverse
consequences on fetal growth and long term outcome,
as suggested by Godfrey et al.5 This would need more
specific definition. Ferritin is not a useful measure as it
falls dramatically in pregnancy irrespective of whether
or not iron supplementation is given.22 Mean corpuscu-
lar volume usually remains fairly constant,22 however,
and might prove to be a more robust indicator,
although it can be confounded by the macrocytic effect
of a simultaneous folate deficiency. Studies that link
any variable proposed with fetal and neonatal function
are needed lest we replace one misleading variable with
is a theoretical
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(Accepted 12 December 1994)
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