Magnetic resonance imaging of first trimester pregnancy: expected intrauterine contents in relation to gestational age

Purpose: To determine if and when fetal structures are identifiable during first trimester pregnancy on magnetic resonance imaging (MRI) in women with acute abdominopelvic pain. Methods: Sixty-four first trimester MRI examinations performed for evaluation of abdominopelvic pain were reviewed retrospectively. T2-weighted images were assessed independently by three radiologists for the presence of gestational sac, yolk sac, fetal pole, and limb buds. Mean sac diameter and sac volume were determined, and logistic regression analyses were performed. Criterion values for fetal structures and gestational sac size were calculated using receiver operating characteristic analysis. Results: A gestational sac is present in all but two cases. The yolk sac is never identified. A fetal pole is identified at a gestational age (GA), mean sac diameter (MSD), and gestational sac volume (GSV) of greater than 6.4 weeks, 1.7 cm, and 8.9 mL, respectively. Limb buds are identified at a GA, MSD, and GSV of greater than 8.1 weeks, 3.4 cm, and 27.1 mL, respectively. There is a significant correlation between GA and gestational sac size, including the MSD (r² = 0.85, P < 0.01) and GSV (r² = 0.86, P < 0.01). Conclusions: The gestational sac is readily demonstrated on T2-weighted images, but the yolk sac is not identified on routine MRI. The fetal pole and limb buds are seen on MRI at a similar GA and MSD as with published values using transvaginal sonography.