Abstract
BackgroundThe physiological fall in haemoglobin concentration from the 1st to the 3rd trimester of pregnancy is often quoted as 5 g/L. However, other studies have suggested varying levels of fall between 8 and 13 g/L. We evaluated the change in haemoglobin concentration between the 1st and 3rd trimesters of pregnancy in a multi-ethnic population of pregnant women.MethodsA retrospective cohort analysis of 7054 women with singleton pregnancies, giving birth during 2013–15 in a single urban maternity unit in England.We calculated the changes in haemoglobin concentration from 1st to 3rd trimester using the first trimester haemoglobin as the reference point. The population was stratified into sub-groups to explore any differences that existed within the population.ResultsIn general the fall in haemoglobin concentration was in the order of 14 g/L or 11% of the first trimester value. This fall was consistent for the majority of sub-groups of the population. The fall was lower (7.7%) in the most deprived section of the population, IMD1, but it increased to 11.7% when we restricted that sub-group to pregnant women without health problems during the index pregnancy. Conversely, there was an increase in haemoglobin of 10.2% in women whose first trimester haemoglobin concentration was in the lowest 5% of the total study population. The population fall in haemoglobin was 10.2 g/L (7.8%), after excluding cases above the 95th and below the 5th centiles, and women with a medical and/or obstetric disorder during the pregnancy.ConclusionThe fall in haemoglobin during pregnancy is in the order of 14 g/L or 11% of the first trimester level. This is 2 to 3 times higher than suggested by some guidelines and higher than previously published work. The results challenge the current accepted thresholds for practice, and have broader implications for diagnosis and managment of antenatal anaemia.Tweetable abstractFall in haemoglobin across pregnancy is around 14 g/L (11%) and significantly higher than previously stated in the pregnant population. This poses questions over currently accepted thresholds for anaemia in pregnancy.
Highlights
The physiological fall in haemoglobin concentration from the 1st to the 3rd trimester of pregnancy is often quoted as 5 g/L
The exceptions were women who fell below the 5th population centile in the first trimester. They had an increase in haemoglobin concetration of 10%; women who were above the 95th centile had a fall in haemoglobin of 26.6 g/L (18%) and women falling between the 5th and 95th centiles who had no co-morbid condition a fall of 10.2 g/L (7.8%) (Table 1)
The findings of this study indicate that the fall in haemoglobin concentration during pregnancy appears to be much greater than often described in guidelines, and is of the order of 14 g/L from the first trimester level
Summary
The physiological fall in haemoglobin concentration from the 1st to the 3rd trimester of pregnancy is often quoted as 5 g/L. It is commonly stated that during pregnancy, the physiological fall in haemoglobin concentration (Hb) is approximately 5 g/L This finding has been based upon changes to the lower 5th centile of haemoglobin concentration between the first and third trimesters and established guidelines that include Hb thresholds for diagnosisng iron deficiency anaemia in pregnancy [1,2,3]. Some older studies have derived reference curves from other published data, while others have charted the changes and found falls in haemoglobin concentration ranging between 8 to 13 g/L. These studies are often limited to a highly selected small samples of pregnant women [4–10]. The change of 5 g/L is reflected in many guidelines, which state the threshold for the diagnosis of anaemia in the second and third trimesters should be lowered from 110 g/L to 105 g/L
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