Determination of blastocyst implantation site in spontaneous pregnancies using three-dimensional transvaginal ultrasound.

Three-dimensional transvaginal ultrasound was used to determine the location of early gestational sacs (E-GSs) and, as a result, the physiological implantation sites of human blastocysts. We examined 138 patients who were found by three-dimensional transvaginal ultrasound to have a GS with an inner diameter of 3 to 6 mm (E-GS). The uterine cavity was divided into three parts: upper, middle, and lower regions. The upper region was subdivided into the right, middle, and left areas, and the middle region was subdivided into the right and left areas. Thus, overall 6 areas were designed and the frequency of the E-GS detection in each area was evaluated. Of the 138 patients, 123 (89.1%) had E-GSs detected in the upper region, which was found to be the most frequent region. When the frequency of E-GS detection among the upper three areas was compared, the right and left upper areas had a higher frequency than the middle upper area. As to the miscarriage rate, patients with E-GSs detected in the upper region had a significantly lower rate than those in the middle and lower regions (p<0.05). The endometrium suitable for human blastocyst implantation under physiological conditions is at the uterine fundus, especially near the uterotubal junction.