Longitudinal Ultrasound Imaging of an Individual Fetus In Utero Using an Echogenic Marker

High frequency ultrasound imaging has become an important tool for the study of pregnancy disorders in mouse models as it enables detailed, noninvasive characterization of in utero development. However, the need to reliably identify individual fetuses over time presents a challenge when using this approach for longitudinal assessments. Large litter sizes and frequently changing fetal positions within the abdomen across gestation have the potential to confound such measurements.

The current study presents a minimally invasive method to longitudinally track a specific mouse fetus through late gestation using ultrasound-guided intra-amniotic injections of an echogenic marker comprised of glass microspheres and shear thinning biomaterial, on embryonic day (E)14.5. Longitudinal measurements of the umbilical artery blood flow were gathered to assess feasibility of a plausible experiment in fetal research. Fetal lung volumes were collected to assess safety of the intra-amniotic sac injection and procedure as amniotic fluid loss during injection risks underdeveloped lungs.

Using CD-1 strain mice, we show that key diagnostic parameters such as umbilical artery velocity and vessel diameter, can be repeatedly measured in the tracked fetus from E14.5 through to term (E18.5). The method is safe for the fetus and pregnant dam with no increased mortality.

Combining this method of mouse fetal tracking with established mouse models of pregnancy-related and gestational conditions proves to be a reliable method for longitudinal data collection in mouse fetuses.