Strengths and Limitations
The major strength of this study is its longitudinal design. The current study is possibly the first study to describe the physiological changes in sHLA-G levels during pregnancy to post delivery in the North Indian females. This is a case-control study nested within a unique pregnancy cohort Garbh-ini. The biospecimen from enrolled participants are being collected in a longitudinal manner and well curated data on multiple clinical variables is being documented. We were able to study associations in a homogenous population with a well-defined phenotype of SGA. The clinical data available on multiple variables allowed us to exclude potential confounders in our study. Instead of using traditional generalized linear models, we used linear mixed model to explore and compare the trajectories of sHLA-G in mothers delivering SGA vs AGA infants. The traditional methods can handle only balanced repeated measure designs. A linear mixed model can handle missingness in the data (unbalanced repeated measure designs), which our study had because of unavailability of the sera sample at postpartum (42 days to 6 months) or because sHLA-G levels were undetectable in some mothers at certain time-points (26 to 28 weeks) of pregnancy and at delivery. This model was robust enough to handle the subject-specific variability which was generated in our study because all the mothers were not sampled at exactly the same day of pregnancy. Inclusion of a post-partum time point in this study reflects the levels of sHLA-G at pre-pregnancy or non-pregnant state. The main limitation of the study is its small sample size and hence the results should be cautiously interpreted.