Ossification timing of sacral vertebrae by ultrasound in the mid-second trimester of pregnancy.

Objectives: The aim of the study was to establish the ossification timing of sacral vertebrae by ultrasonography in the second trimester of pregnancy, for the diagnosis of caudal regression syndrome with isolated sacral agenesis.

Methods: The study was carried out on 77 normal single pregnancies, at gestational ages ranging from 15 to 21 weeks, using high-resolution transabdominal echography. The sacral region was visualized in a coronal plane, when the fetus was in anterodorsal position. The level of ossification of sacral vertebrae (S1 to S5) at each gestational age was recorded. Each sacral region was examined three times by the same observer and the nucleus was considered as present when it was visualized at least two times out of three. Blind assessment was performed three times by a second observer, who was not present at the previous examination, for interobserver and intraobserver error analysis.

Results: Interobserver and intraobserver error calculation demonstrated the reproducibility of the method. Concordance between the two observers as evaluated by Cohen Kappa index was 0.77 (C.I. 95%, 0.69-0.85).S1 ossification nuclei were visualized in all fetuses at 15 weeks and S2 nucleus was found in all fetuses within 17 weeks. S3 nucleus was detected in 45% of fetuses by the beginning of the 16th week. S4 was visualized in 55% of the cases at 18 weeks and progressively in a higher percentage of cases during the following weeks of gestation.

Conclusion: The data obtained showed that the sequence of development of sacral region ossification was related to gestational age. This observation allows clinicians to accurately exclude isolated sacral agenesis at 16 to 17 weeks of gestation, when the S1-S2 ossification nuclei are visualized. This opportunity may be of particular value in the offspring of diabetic mothers.