 In 1747, French obstetrician Andre Levret, published "Observations sur les causes et accidents de plusieurs accouchements laborieux" ("Observations on the Causes and Accidents of Several difficult Deliveries"), in which he described his modification of the instrument to follow the "curvature" of the maternal pelvis, this "pelvic curve" allowing a grip on a fetal head "still high" in the pelvic excavation, which could assist in more difficult cases.This improvement was published in 1751 in England by William Smellie in the book "A Treatise on the theory and practice of midwifery".After this fundamental improvement, the forceps would become a common obstetrical instrument for more than two centuries.