The participants were healthy nulliparous women with singleton pregnancies recruited to the Screening for Pregnancy Endpoints (SCOPE) study between November 2004 and July 2007 in Auckland, New Zealand, and Adelaide, Australia.10
SCOPE is a prospective, multicentre cohort study with the main aim of developing screening tests to predict pre-eclampsia, small for gestational age infants, and spontaneous preterm birth.
Women were recruited to the SCOPE study by 15 weeks’ gestation through hospital antenatal clinics, obstetricians, general practitioners, community midwives, and self referral in response to advertisements or recommendations of friends. Women were excluded if they were judged to be at high risk of pre-eclampsia, small for gestational age babies, or spontaneous preterm birth because of underlying medical conditions, gynaecological history, three or more previous miscarriages, or three or more terminations of pregnancy, or who had received interventions that might modify pregnancy outcome.10
A research midwife interviewed and examined women who agreed to participate at 15 and 20 weeks’ gestation. At the interview women also completed a lifestyle questionnaire incorporating the following psychological scales: the short form of the state trait anxiety index, measuring anxiety;11
the perceived stress scale, measuring perceived stress;12
and the Edinburgh postnatal depression scale, measuring depression.13
We use data from the 15 weeks’ interview in this publication. At the time of interview, data were entered into an internet accessed, central database that has a complete audit trail. Participants were followed up prospectively, with pregnancy outcome data and baby measurements collected by research midwives, usually within 72 hours of birth. Stringent data monitoring included individual checking of all data for each participant (including checking for any data entry errors in the lifestyle questionnaire) and using a customised software program to detect any systematic data entry errors.
Participants were divided into three groups, according to self reported maternal smoking status, which was ascertained by direct questioning at recruitment to the SCOPE study at 15 weeks’ gestation. “Non-smoker” referred to women who did not smoke at any time during pregnancy, “stopped smoker” to women who had smoked at some time during pregnancy but had stopped before the 15 weeks’ SCOPE interview, and “current smoker” to women who continued to smoke at the 15 weeks’ interview. Stopped smokers were asked at what gestation they stopped smoking, and stopped and current smokers were asked about the number of cigarettes smoked daily in the three months before pregnancy, in the first trimester, and at the 15 weeks’ SCOPE visit, as appropriate. Smoking status was not confirmed using biochemical measures.
The primary outcome measures were spontaneous preterm birth and small for gestational age by customised centiles. Secondary outcome measures were birth weight, gestation at delivery, uncomplicated pregnancy, and measures of stress, anxiety, and depression.
The estimated date of delivery was calculated from a certain last menstrual period date. The estimated date of delivery was only adjusted if either a scan at less than 16 weeks’ gestation found a difference of seven or more days between the scan gestation and that calculated by the last menstrual period, or at a 20 week scan a difference of 10 or more days was found between the scan gestation and that calculated from the last menstrual period. If the last menstrual period date was uncertain, scan dates were used to calculate the estimated date of delivery. Small for gestational age was defined as birth weight below the 10th customised centile adjusted for maternal weight, height, parity, ethnic group, and infant sex (www.gestation.net
Spontaneous preterm birth was defined as spontaneous preterm labour or preterm premature rupture of the membranes resulting in preterm birth at less than 37 weeks’ gestation. Pre-eclampsia was defined as gestational hypertension (systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg on at least two occasions four hours apart after 20 weeks’ gestation, but before the onset of labour, or postpartum systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg on at least two occasions four hours apart) with proteinuria (24 hour urinary protein ≥300 mg, or spot urine protein to creatinine ratio ≥30 mg/mmol creatinine, or urine dipstick protein ≥2+) or any multisystem complication of pre-eclampsia.15
Uncomplicated pregnancy was defined as a pregnancy with no antenatal obstetric or medical complications and resulting in delivery of an appropriately grown, healthy baby at 37 or greater weeks’ gestation.
We used ANOVA tests to compare continuous variables between the three smoking groups with post hoc Tukey test for pairwise comparisons, Student’s t test was used to compare continuous variables between stopped smokers and non-smokers and between stopped smokers and current smokers, and χ2 or Fisher’s exact tests to compare categorical variables, as appropriate.
We used logistic regression to compare the odds of spontaneous preterm birth and small for gestational age between stopped smokers and non-smokers and between stopped smokers and current smokers, adjusting for potential confounders. Potential confounders included demographic factors (age, self assigned ethnic group, marital status, employment, and body mass index), risk factors suggested in previous publications (bleeding during pregnancy, folic acid use, use of multivitamins or alcohol at 15 weeks gestation, and depression, stress, and anxiety scores at 15 weeks’ gestation). The logistic regression model for spontaneous preterm birth also included history of previous miscarriage or termination of pregnancy and large loop excision of the cervical transformation zone. All variables were obligatory data points and were available for all of the participants, with the exception of scores for the psychological scales, which were missing in 18 (0.7%) of the participants. We excluded data for these 18 women from the logistic regression models.