Participants

Participants were pregnant women recruited from the antenatal clinic of Kwong Wah Hospital, a public hospital managed by the Hospital Authority in Hong Kong. Of note, public hospitals in Hong Kong are highly subsidized and provide ~80% of in-patient services in the region. Kwong Wah Hospital has one of the city’s major obstetrics and gynaecology departments, providing services to ~5000 child births annually. During the study period (~3 months in 2017), we recruited 774 pregnant women who met the study’s inclusion criteria: (1) were age 18 years or above, (2) gave their informed consent, and (3) were in the 20th–24th week of gestation at the time of recruitment. At the time of recruitment, the mothers’ report of any history of exposure to IPV, demographic characteristics, health-related quality of life, engagement in risk behaviors, and mental health symptoms such as depression, stress, and anxiety were collected. Umbilical cord blood samples from the newborns were subsequently collected by midwives during delivery. The cord blood samples were used for assessing newborn TL. The mothers were contacted 4 weeks after delivery. The mothers reported again about their history of exposure to IPV at any time in their lives up to childbirth and their obstetric outcomes.

Measures

Outcome measure

Newborn telomere lengths

Samples of the newborns’ DNA were extracted from the umbilical cord blood collected by midwives during each baby’s delivery. Genomic DNA samples were isolated and extracted from the collected samples using the QIAamp DNA Mini kit (Qiagen), according to the manufacturer’s instructions. Isolated DNA samples were eluted to the buffer solution (10 mM Tris-HCl and 1 mM ethylenediamine-tetraacetic acid, pH 8.0) for quality checking and quantification by spectrophotometry (NanoDrop 2000c, Thermo Scientific). Upon confirming that the DNA quality and quantity were in the acceptable range for the determination of telomere length, each sample was processed following the procedures described in previous literature22. Each sample was handled in triplicate for the telomere length assay by quantitative polymerase chain reaction (qPCR) using a 7900HT Thermocycler (Applied Biosystems). After the assay, the telomere length was presented by a relative ratio of the telomere repeat copy number (T) to single-copy gene 36B4 copy number (S) using the formula of T/S = 2(−ΔCt), where ΔCt is the mean difference between the threshold cycle (Ct) value of the 36B4 gene and telomere repeats obtained from the qPCR performed.

Predictor

IPV against women by current partners before childbirth

The mothers reported on their exposure to violence by their current partners in the form of psychological, physical, and sexual abuse at any time in their lives up to childbirth, using the five-item Abuse Assessment Screen (AAS)23. The AAS has been commonly used in different healthcare settings, and the validated Chinese AAS demonstrated satisfactory measurement accuracy for identifying IPV against women when it was used to evaluate Chinese women in a previous study24. The mothers who reported any history of exposure to IPV at any time in their lives up to the time of childbirth were categorized into “abused group” and those who did not have any history of exposure to IPV were categorized into “non-abused group.”

Covariates

Maternal anxiety and stress symptoms

These were assessed using the anxiety and stress subscales of the Chinese version of the Depression Anxiety Stress Scale (DASS)25. The DASS has been widely used in research and clinical settings. The anxiety and stress subscales each consist of seven items that assess the severity (identified as normal, mild, moderate, and severe) of respective core symptoms. The DASS has been validated in a Chinese population26. The symptom scores were adjusted in the regression models as continuous variable.

Maternal depressive symptoms

Maternal depressive symptoms were assessed using the 10-item Edinburgh Postnatal Depression Scale (EPDS)27. The EPDS assesses the presence of maternal depressive symptoms during the perinatal period. The mothers were asked to report the presence of depressive symptoms. The Chinese version of EPDS has been validated in a previous study28. The symptom scores were adjusted in the regression models as a continuous variable.

Maternal health-related quality of life

The Chinese version of the Short-Form Health Survey, version 2 (SF-12)29 was used to assess the mothers’ maternal health-related quality of life (HRQoL). The SF-12 consists of 12 items that measure eight health domains, including physical functioning, role limitations due to physical health, bodily pain, general health, vitality, social functioning, role limitations due to emotional health, and mental health. The SF-12 generates two composite scores, the physical component summary (PCS) and the mental component summary (MCS), which were adjusted in the regression models as continuous variables.

Maternal risk behaviors

Maternal risk behaviors, including gambling, cigarette smoking, alcohol drinking, and drug use were assessed using four items. The mothers were asked to report how often they engaged in each of those risk behaviors, by rating them as 1 = never; 2 = rarely; 3 = sometimes; and 4 = always. Because these risk behaviors were not common in this sample, the variables were grouped as binary variables (never vs. ever engaged in the behavior) in the analysis.

Demographic characteristics and obstetric factors

Demographic information about the mothers, such as maternal age, educational level, marital status, family income, self-reported obstetric problems (hyperemesis gravidarum, hypertension, diabetes mellitus, multiple pregnancy, and vaginal bleeding), gestation age, and birth weight were collected and considered in our analyses. Maternal age, family income, gestation age, and birth weight were adjusted as continuous variables, while education level, marital status, and obstetric problems were adjusted as categorical variables.

Data analysis

Descriptive statistics of the participants’ characteristics and exposure to IPV before childbirth were first computed. To examine the association between IPV against the women before childbirth and TL in their newborns, four regression models were generated with increasing number of covariates to examine the influence of adjustment covariates on the association. This approach is to illustrate the robustness of the association regardless of adjustment schemes, rather than for variable selection as in traditional stepwise approach. The statistical significance of the adjustment variables is not of primary interest in this study and serve as a reference only. The first model was of crude associations between different types of IPV against the women before childbirth and their newborns’ TL. In the second model, the associations were examined with adjustments for demographic variables, obstetric problems, and newborn characteristics. The third model additionally included the mothers’ health-related quality of life as the adjusted variables. The fourth model additionally included the mothers’ mental health variables as the adjusted variables. These adjustment variables were selected due to previous reports on their simultaneous association with the exposure (IPV) and outcome (TL). Due to a highly positive skewness of the distribution (2.34), a logarithmic transformation of the TL was performed before the analysis. Data analyses were performed using R Statistical Software. All tests were two-tailed, and P-values below 0.05 were regarded as statistically significant. Multicollinearity was examined using variance inflation factor (VIF) where variables with VIF >10 were removed as a sensitivity analysis.

Ethical approval

The research protocol was approved by the Institutional Review Board of the Hospital Authority Kowloon West Cluster Research Ethics Committee (Reference number: KW/FR-16-042(97-01)(1)).