Radiographic Pregnancy Diagnosis.
Seven skulls and spines can be identified at about 50 days of gestation.
Verstegen J (2009)
Radiography of Fetuses.
Comparison of three fetuses at different stages of gestation ranging from 45 days at the top, 50 days in the middle, and term at the bottom of the image. The density of the 45-day fetus is too light to significantly reflect the X-rays.
Shille VM (1982)