Methods
Study sites and participant selection
procedures
We conducted an institutionally based retrospective cohort study between
July 2017 and January 2018 at 60 public hospitals in China. Study
participants were women who had given birth between 12 and 24 months
earlier. First, we divided the country into three macrogeographic
regions: eastern, central, and western. This is because there are huge
socioeconomic differences and landscape variations among the three
regions: Eastern China is the most developed and located near the
eastern coastline of the country. Western China is the least developed
and dominated by towering massifs and rolling plateaus. Central China is
largely an agricultural area and generally less developed than the
eastern coastal provinces but more developed than the western provinces.
Second, we selected five provinces in each region (see Supplementary
material, Figure S1), and appointed a senior physician or researcher as
the provincial coordinator for each province. Third, we asked the
provincial coordinators to provide a list of 10 hospitals with at least
two each level-2 and level-3 general hospitals and two each level-2 and
2 level-3 Maternal and Child Hospitals, and this defined a sampling
frame. A statistician randomly selected four hospitals from each
province using computer-generated random numbers, randomly selecting a
month between July 2015 and June 2016; and allocated it to each
participating hospital as the initial month of investigation. The first
300 women who delivered a live birth in the selected month were eligible
for this study. If the number of deliveries in the given month was less
than 300, women who delivered in the following month were invited for
interviews.
Sample size calculation
The sample size was calculated based on a prevalence (P) of unintended
pregnancy of 8% among postpartum women in a given region. With a 0.01
uncertainty level (d), 95% confidential interval (CI), and 5% margin
of error, and using the formula N =Z2 0.05P(1-P)/d2,
we calculated a required sample size of 2828. As we used a cluster
(hospital) sampling technique to select the participants (as proposed by
Donner, Birkett, and Buck), the sample size calculated assumed that
individual randomization could be inflated by a design effect (DE) to
reach the required level of statistical power under cluster
randomization: DE = 1 + (n-1)ρ, where n was the number of individuals
per cluster and ρ was the intracluster correlation coefficient
(ICC).(15) When the primary outcome variable is a
rate, the coefficient of variation in the outcome can be used as an
alternative measure to the ICC, and is denoted by
κ(16) (κ is usually derived from previous studies).
However, we did not find any relevant studies where investigators
reported the value of κ. Based on experience, then, we set DE = 2,
assuming a κ of 0.003 (using data from this study, we derived a κ of
0.001). We thus obtained a sample size of 5656 for each geographic
region, or 283 participants for each hospital. We decided to recruit 300
participants at each hospital, which allowed for a 6% non-response
rate.
Data collection
Data were collected with a structured questionnaire via face-to-face or
telephone interviews. Two junior doctors and/or nurses selected as data
collectors at each hospital were trained on the data collection
procedures and the interview techniques. Recorded information included
participants’ background characteristics, pregnancy history, immediate
postpartum contraceptive use, resumption of sexual activity, return of
menstrual cyclicity, and occurrence and time of subsequent pregnancy
and/or induced abortion after childbirth. All of the participants were
interviewed between August 2017 and January 2018.
Data processing and
analysis
We defined long-acting reversible contraceptives (LARCs) as intrauterine
devices (IUDs) and implants. Modern contraceptive methods were defined
as using one or more of the following method(s): male and female
sterilizations, male and female condoms, combined oral pills (COCs),
injectables, implants, IUDs, emergency contraceptive pills (ECPs), and
spermicides. Traditional methods were defined as the use of the
lactational amenorrhea method (LAM), rhythm/calendar method, and
withdrawal. We defined unmet need for postpartum contraception as women
who resumed sexual intercourse after delivery but did not use any
contraceptive methods or only used traditional methods.
Data were entered into EPI INFO version 3.1, and we used SAS 9.4 (SAS
Institute Inc.) for data analysis. Frequencies, percentages, means +/-
SD, and cross-tabulations were used to summarize descriptive statistics.
Life-table analyses were used to calculate cumulative rates of
contraceptive use, unintended pregnancy, and induced abortion during the
first 24 months postpartum. We used the log-rank test to compare trends
and Chi-squared test to compare rate differences between/among regions.
Ethical considerations
Ethical approval was obtained from the Institutional Review Board of
Shanghai Institute of Planned Parenthood Research. Written (face-to-face
interviews) and verbal (telephone interviews) consent was obtained from
all the study participants, and they were assured that the information
would remain confidential.