 There is no published data on the rate of B12 deficiency in GDM women in the UK and limited data exists in non-GDM women in Caucasians. Preliminary results from the investigators group have shown that approximately 15 insufficiency in non-GDM and around 20-25 in GDM women of Caucasian origin. To detect a 5 difference in the prevalence of B12 insufficiency with 90 power at the 5 significance level, 700 GDMs will be required. We plan to identify 1400 controls matched for age, BMI and parity. To achieve this,4080 pregnant women will be required in their first trimester over the 2.5 years if 20 develop GDM. Allowing for a 10 drop-out rate and for age, parity BMI matching 4500 pregnant mothers will be recruited. 