Objective Using laboratory reference ranges, B12 deficiency is inappropriately diagnosed and treated in pregnancy. The authors aim to define reference ranges for ferritin, folate, haemoglobin and B12 in a pregnant population with advancing gestation.

Design A cross-sectional study was performed between May 2008 and January 2009.

Setting Ayrshire Maternity Unit and peripheral clinics within Ayrshire and Arran.

Methods After obtaining informed consent, venesection was performed at booking and 34 weeks in healthy pregnant women. Information regarding demographic details, dietary influences and use of supplements was obtained. Patients with ‘low’ B12 in the absence of anaemia and/or macrocytosis were not treated and invited to return for postnatal review.

Results 190 women participated, 113 in the first and 77 in the third trimester. All variables studied except red cell folate, decreased significantly from the first to third trimester. 34% (64/190) of women were found to have ‘low’ B12 as defined by traditional ranges. In women with anaemia and apparent B12 deficiency, coexisting ferritin deficiency was demonstrated. All B12 levels spontaneously increased postnatally.

Conclusion The authors propose that use of gestation specific reference ranges for haematological variables, as defined by this study, would reduce inappropriate diagnosis of B12 deficiency. In most women with apparent low vitamin B12 levels and anaemia, ferritin deficiency was demonstrated and therefore iron should be the initial management therapy.