Ultrasound-Guided Intrauterine Labeling of Rat Fetuses

Aim: To compare intra-amniotic versus fetal subcutaneous injections for selective fetal labeling in multifetal rat pregnancies. Methods: A total of 14 pregnant rats were randomized to receive intra-amniotic injections of dyes (including Fluorescein, Indigo Carmine, or Evans Blue) or fetal subcutaneous injections (of commercial tattoo ink) both guided by ultrasound at 15-17 days of gestation. Survival, injection, and labeling success rates of both techniques were compared. Results: Survival rates (84.4% for intra-amniotic injections vs. 90.9% for fetal subcutaneous injections) and injection success rates (94% for intra-amniotic injections vs. 100% for fetal subcutaneous injections) were similar among both groups. None of the neonates from the intra-amniotic injections group were labeled at birth, while 93% of the neonates from fetal subcutaneous injections group were tagged, showing a visible spot in the skin at birth. Conclusion: Our results suggest that ultrasound-guided fetal subcutaneous injections might be an adequate strategy for selectively labeling fetuses in multifetal pregnant animals.