© 2019 Taylor & Francis Group, LLC.Objective: We compared markers of oxidative stress (OS) in mothers with and without fetal neural tube defects (NTDs). Methods: Pregnant mothers in the second trimester with NTD-affected fetuses and age, gestational age, and body mass index-matched control mothers with unaffected fetuses were included. Maternal serum thiol-disulfide homeostasis parameters and ischemia-modified albumin (IMA) were measured. Results: In 30 affected mothers compared to 31 controls, disulfide levels, disulfide/native thiol, and disulfide/total thiol ratios were higher; native and total thiol levels and native thiol/total thiol ratios were lower (p < 0.001). Mothers with NTD-affected fetuses had higher levels of IMA than controls (p = 0.025). Conclusion: The thiol-disulfide homeostasis balance was shifted in favor of disulfide, suggesting increased thiol oxidation and OS in the second trimester of NTD-affected pregnancies. Maternal levels of IMA, an oxidatively altered form of albumin, thus a measure of OS, were higher in NTD-affected second trimester pregnancies compared to controls.