Profitable production of pork depends to a significant extent on the number of piglets born and raised by each gilt or sow introduced into the breeding herd. A sow that does not conceive or has a small litter will reduce potential profit. Determination of pregnancy by ultrasonic echo amplitude is quite accurate from about day 30 to day 55 but gives essentially no indication of potential litter size. Analysis by real time ultrasound later in gestation can give some indication of the potential litter size but is expensive, requires much time and a certain measure of skill. Each fetus produces hormones as early as day 15. These hormones can be detected in the sow=s blood beginning about day 18. The concentration of the hormones rises very rapidly from day 18 to day 28 and then declines sharply. Each fetus adds to the hormone concentration so that the concentration is influenced not only by the day of gestation on which a blood sample is taken but also on the number of fetuses contributing to the concentration of the hormone. Estrone glucuronide (E1 G) is the hormone that is present in the greatest concentration and is easiest to measure and is related to pregnancy and litter size. Several studies were conducted to understand the relationships between day of gestation, pregnancy and litter size