Data source and variables
This was a retrospective cohort study that used the ART databases from
two affiliated hospitals of Peking University (Peking University Third
Hospital and Peking University Shenzhen Hospital). 24,422 frozen-thawed
embryo transfer (FET) cycles recorded from January 2015 to May 2018 were
selected to generate the research database. The database included the
following; maternal characteristics (maternal age at treatment, height,
weight, infertility type, cause of infertility and duration of
infertility, and number of previous ART cycles), treatment records of
the current cycle (type of fertilization in the current cycle, numbers
of embryo transferred, days of embryo development), maternal outcomes
(biochemical pregnancy, clinical pregnancy, monozygotic twins,
miscarriages, and maternal complications) and neonatal outcomes (live
births, number of live births, gestational age at delivery, birth
weight, and congenital malformations).
Of the 24,422 FET cycles, 440 preimplantation genetic analysis (PGA)
cycles, 42 in vitro maturation (IVM)
cycles, 3 assisted oocyte activation (AOA) cycles, 13 cycles with
missing values in numbers of embryo transferred, 6 cycles with missing
values in days of embryo development, 20 cycles with two embryos in
different development days and 172 cycles with loss to follow-up were
excluded from the analysis. Finally, 23,726 cycles were included for the
final analysis. This included 13,767 cleavage-stage embryo transfer
cycles (58.0%) and 9,959 blastocyst-stage embryo transfer cycles
(42.0%). Based on the stage (cleavage-stage or blastocyst-stage) and
number (one or two) of embryos transferred, all the selected cycles were
divided into four exposure groups: the single cleavage-stage embryo
transfer group (C-1) (763 cycles, 3.2%), double cleavage-stage embryo
transfer group (C-2) (13004 cycles, 58.0%), single blastocyst-stage
embryo transfer group (B-1) (7913 cycles, 33.4%) and double
blastocyst-stage embryo transfer group (B-2) (2046 cycles, 8.6%).
(Figure 1)
The following data were analyzed for each cycle:
maternal
age (20-29 years, 30-34 years, 35-37 years, 38-39 years, 40-42 years, or
>42 years), body mass index (BMI) (equal to height/weight2
(m/kg2), BMI<18.5 defined as ‘underweight’,
18.5≤BMI<24 defined as ‘fitting’, 24≤BMI<27 defined
as ‘overweight’, or BMI≥27 defined as ‘obesity’), infertility type
(primary infertility, or secondary infertility), cause of infertility
(tubal, ovulatory, endometriosis, male factor, or unexplained), duration
of infertility (≤4 years or >4 years), type of
fertilization in the current cycle (IVF, ICSI, or IVF+ICSI), previous
ART cycles (0-1 cycles, 2-3 cycles, 4-5 cycles, or ≥6 cycles),
biochemical pregnancy (yes or no), clinical pregnancy (yes or no),
monozygotic twins (yes or no), miscarriage (yes or no), maternal
complications (yes or no), live birth (yes or no), twin birth (yes or
no), preterm birth (defined as ‘gestational week at delivery
<37 weeks, yes or no), low birth weight (defined as ‘birth
weight at delivery <2500 grams’, yes or no), small for
gestational age infant (SGA, defined as ‘birth weight at delivery
<10th percentile for the baby’s gender and gestational age,
using Chinese reference
charts’3, yes or no),
and congenital malformations (yes or no). With regards to cycles with
multiple pregnancies or births, the cycle was defined as “yes” for
live births, preterm births, low birth weights, SGA or congenital
malformations.
The study was approved by the Peking University Third Hospital Medical
Science Research Ethics Committee (IRB00006761-M2019107).