Placenta tissue collection and histopathology
For study participants in Ottawa, placentas were collected at the time of delivery and sent to Pathology. Trimmed placental weight was recorded and gross pathology was recorded by an experienced Pathology Assistant. Four full thickness tissue biopsies were randomly excised from each quadrant of the placenta, between the cord insertion site and the placental margins. Areas of visible pathology were sampled separately and not used for the full thickness sections. Tissue was fixed in 4% neutral buffered formalin and paraffin-embedded. Following sectioning (5um), tissue was stained with hematoxylin and eosin (H&E) using standard protocol32 and stored for examination. In Kingston, archived H&E-stained tissue slides (4-5 slides/participant) were accessed from Pathology archives for each participant. Sampling procedures were similar to those followed in Ottawa in that full thickness biopsies were excised from each quadrant of the placenta, between the margin and cord insertion site. Trimmed placenta weight and gross pathology were collected from accompanying placental pathology reports. A single, experience placental pathologist examined the stained slides from all study participants, blinded to all clinical information apart from gestational age at delivery. Placental lesions were evaluated according to a standardised placental pathology data collection form, with pre-specified severity criteria derived from clinical practice guidelines and published literature.33 Within the evaluation scheme, each lesion has a severity score to achieve a quantitative output for the severity of pathology. Lesions were either given a binary score of 0 (absent) or 1 (present) or a graded score according to a liner scale (i.e. 0 = absent, 1 = focal, 2 = patchy, 3 = diffuse). Individual lesions are grouped according to broad etiological categories, with a maximum severity score calculated for each category. Lesion categories and maximum severity score for each category are as follows: MVM (max score 13), implantation site abnormalities (max score 4), histological chorioamnionitis (max score 11), placental villous maldevelopment (max score 5), fetal vascular malperfusion (max score 6), chronic utero-placental separation (max score 3), maternal-fetal interface disturbance (max score 5) and chronic inflammation (max score 6). Gross anatomy (e.g., placental weight, umbilical cord length) was obtained from the corresponding historical placental pathology reports, in addition to several microscopic lesions (e.g., placental infarction, chronic deciduitis), as the tissue biopsies were collected from areas that appeared grossly normal and only included villous parenchyma (i.e., maternal decidua was not sampled).