Background A fundamental obstacle to using retroviral‐mediated gene transfer (GT) to treat human diseases is the relatively low transduction levels that have been achieved in clinically relevant human cells. We previously showed that performing GT in utero overcomes this obstacle and results in significant levels of transduction within multiple fetal organs, with different tissues exhibiting optimal transduction at different developmental stages. We undertook the present study aiming to elucidate the mechanism for this age‐dependent transduction, testing the two factors that we hypothesized could be responsible: (i) the proliferative status of the tissue at the time of GT and (ii) the expression level of the amphotropic PiT‐2 receptor. Methods Immunofluorescence was performed on tissues from sheep of varying developmental stages to assess the proliferative status of the predominant cells within each organ as a function of age. After developing an enzyme‐linked immunosorbent assay (ELISA) and a quantitative reverse transcription chain reaction (qRT‐PCR) assay, we then quantified PiT‐2 expression at the protein and mRNA levels, respectively. Results The results obtained indicate that the proliferative status of organs at the time of fetal GT is not the major determinant governing transduction efficiency. By contrast, our ELISA and qRT‐PCR analyses demonstrated that PiT‐2 mRNA and protein levels vary with gestational age, correlating with the observed differences in transduction efficiency. Conclusions The findings of the present study explain the age‐related differences that we previously observed in transduction efficiency after in utero GT. They also suggest it may be possible to achieve relatively selective GT to specific tissues by performing in utero GT when levels of PiT‐2 are maximal in the desired target organ. Copyright © 2012 John Wiley & Sons, Ltd.