Objectives: Many women report declines in cognitive function during pregnancy, but attempts to empirically evaluate such changes have yielded inconsistent results. We aimed to determine whether pregnancy is associated with objective declines in cognitive functioning, and to assess the progression of any declines during pregnancy. Study design: We undertook a meta-analysis, applying a random effects model, of 20 studies that have reported quantitative relationships between pregnancy and changes in cognition. Data sources: Full text articles indexed by Cumulative Index to Nursing and Allied Health Literature (CINAHL) Complete, MEDLINE Complete, and PsychINFO. Data synthesis: The 20 studies assessed included 709 pregnant women and 521 non-pregnant women. Overall cognitive functioning was poorer in pregnant women than in non-pregnant women (standardised mean difference [SMD], 0.52 [95% CI, 0.07-0.97]; P = 0.025). Analysis of cross-sectional studies found that general cognitive functioning (SMD, 1.28 [95% CI 0.26-2.30]; P = 0.014), memory (SMD, 1.47 [95% CI, 0.27-2.68]; P = 0.017), and executive functioning (SMD, 0.46 [95% CI, 0.03-0.89]; P = 0.036) were significantly reduced during the third trimester of pregnancy (compared with control women), but not during the first two trimesters. Longitudinal studies found declines between the first and second trimesters in general cognitive functioning (SMD, 0.29 [95% CI, 0.08-0.50]; P = 0.006) and memory (SMD, 0.33 [95% CI, 0.12-0.54]; P = 0.002), but not between the second and third trimesters. Conclusions: General cognitive functioning, memory, and executive functioning were significantly poorer in pregnant than in control women, particularly during the third trimester. The differences primarily develop during the first trimester, and are consistent with recent findings of long term reductions in brain grey matter volume during pregnancy. The impact of these effects on the quality of life and everyday functioning of pregnant women requires further investigation.