 Spot urine samples were collected from the SWS women at clinic visits following their initial interviews median 33 IQR 15-87 days, in the period before conception. The time of day was recorded for each urine collection. Urine samples were split divided into aliquots on the day of collection. They were frozen and stored at -800oC from the date of collection until being thawed and assayed for iodine and creatinine contents in 2016 by the Trace Element Unit, Southampton General NHS Trust Hospital, Southampton, UK. 