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).