PROJECT SUMMARY/ABSTRACT Iodine deficiency has increased dramatically in the U.S. in recent decades leading to over 40% deficiency in reproductive age women. Moreover, infertility and pregnancy loss are increasingly common with far reaching physical, social, and emotional consequences. Targeted low-cost interventions are needed to address these adverse reproductive outcomes. Iodine is a potential therapeutic for these outcomes as it is biologically important for critical hormones involved in reproduction, placentation, and growth and development of offspring. Severe deficiency is well known for its effect on hypothyroidism and irreversible brain damage in offspring of deficient pregnant women. However, little is understood on the effects of deficiency on women's fertility, fecundity, pregnancy loss, and other pregnancy complications. Research into the implications of deficiency has proven quite challenging as it does not always present clinically and is incredibly variable day- to-day. It is commonly recommended that 10 urine samples are needed to accurately classify iodine status of an individual. Yet many studies fail to follow this recommendation and rely on a single spot urine which can lead to grossly attenuated findings, hampering progress in iodine research. An accurate evidence base is critically needed to inform policy and individual decisions on iodine fortification, supplementation, and dietary intake, all which of which offer a potential low-cost solution to promote the health and wellbeing of women and their offspring. We propose a novel application of biospecimen pooling to classify individual's iodine status and overcome prior measurement challenges. We pooling have developed an innovative hybrid design that uses both and random sampling to measure iodine in preconception and throughout pregnancy.The overarching goal of this study is to investigate the role of iodine on reproductive and perinatal outcomes including rarely studied outcomes of fecundability, fertility, very early pregnancy loss. This work will utilize the EAGeR study (n=1228), an existing well-defined, preconception cohort of women at high risk for pregnancy loss with daily urine collection preconception, and if they achieved pregnancy, during the first four weeks of pregnancy with additional spot urine samples throughout, and collection of the placenta at delivery. Using novel biospecimen pooling methods we will classify iodine status in preconception, early-, mid-, and late- pregnancy. Using a trial emulation approach, we will investigate the effects of iodine deficiency, across critical periods of susceptibility on reproductive and perinatal outcomes. We will also measure iodine in placenta samples; little is known about how maternal iodine in gestation relates to placental iodine and how this interrelationship relates to pregnancy outcomes. The successful completion of these aims with the novel characterization of iodine status from preconception through gestatio...