Study selection, data collection and data items
All studies (cohort studies and case-control studies) reporting data on
the birthweight discordance affected by GHD in twin pregnancies were
included. This review included all women in
both DC and MC twin pregnancies
complicated with gestational hypertension, chronic hypertension,
preeclampsia and eclampsia. We excluded pregnancies with intrauterine
death of one or both twins.
The outcome was the intertwin birthweight discordance, which was
calculated utilizing the following equation: birthweight discordance
(%) = (larger twin’s birthweight − smaller twin’s birthweight)/larger
twin’s birthweight) × 100. We performed the review according to one of
the most common cut‐offs of
birthweight discordance (≥15%).4
Furthermore, according to the classification of GHD, we planned to
perform sub-group analyses reporting cases with GH, PE, eclampsia and CH
separately. According to the criteria outlined by
ACOG,32,33 concretely,
GH was defined as
systolic blood pressure (BP) ≥140
mmHg or ≥ 90 diastolic mmHg after 20 weeks of gestation, PE met the same
criteria as GH with either proteinuria or end-organ dysfunction (liver
dysfunction, renal dysfunction, central nervous system disturbances,
thrombocytopenia, or pulmonary edema). Eclampsia was defined as a new
onset of seizures in women with PE, who showed a convulsive episode or
other sign of altered consciousness. CH was diagnosed as a pre-existing
hypertensive disease or BP ≥140 mmHg systolic or ≥ 90 mmHg diastolic
preceding 20 weeks gestation.
In order to ascertain whether the association between GHD and intertwin
birthweight discordance differs according to chorionicity,
stratification by twin chorionicity (DC and MC) was performed. The
assessment of chorionicity was clinically based on the
ultrasound evaluation, including the
number of gestational sacs, placenta and embryos at early pregnancy,
τ-sign or λ-sign, the fetal
sex,34 and after delivery it was confirmed by
examining the placenta.
Two authors (YW, HYZ) screened all studies independently in view of the
inclusion and exclusion standards. Where there were inconsistencies,
consensus was reached by discussion with another reviewer (FZ). With
full text copies of included studies obtained, the same
two reviewers extracted relevant
data in regard to study characteristics and pregnancy outcomes
independently. In case over one study was published based on the
identical cohort with uniform endpoints, the report demonstrating the
most comprehensive information was included.
Quality assessment was performed with the Newcastle-Ottawa Scale (NOS)
adapted for cohort and case-control studies. Each study is evaluated
from three broad aspects: selection of patients, comparability of study
groups and ascertainment outcome of interest. The scores of the NOS
range from 0 to 9. The included studies were interpreted to be of high
risk of bias (for scores ≤ 4), medium risk of bias (for scores 5-7), or
low risk of bias (for scores ≥ 7).35 Articles with low
risk of bias were included in the analysis.