High-resolution transvaginal ultrasonography may recognize structures in the first- and early second-trimester fetus. In a patient with pregestational diabetes, caudal regression syndrome in the fetus was diagnosed using transvaginal ultrasonography. At 9 weeks of gestation, a shortening of the crown-rump length and a protuberance of the lower spine suggested caudal regression syndrome. By 17 weeks of gestation, the diagnosis was made with certainty. The transvaginal approach has changed the role of first-trimester ultrasound in the diabetic pregnancy. We suggest that transvaginal ultrasonography be used for purposes of accurate dating and for early detection of diabetic embryopathy, particularly in patients with poor periconceptional glycemic control.