Study design and population
From 2017 to 2020, the TSBC enrolled women aged 18-40 years with a singleton pregnancy in the Shuangliu Maternal and Child Health Hospital in Chengdu, China. Details of this birth cohort have been reported elsewhere 28. Briefly, pregnant women who attended antenatal care and had an gestational age ≤15 weeks were invited to participate in our study. Women who met the following criteria were included: 1) women aged 18-40 with a singleton pregnancy; 2) gestational age less than 15 weeks. Participants were excluded if they 1) conceived the fetus using assisted reproductive technology, such as in-vitro fertilization and intrauterine insemination; 2) reported severe chronic disease or infectious disease like cancer, tuberculosis, and HIV infection; 3) refused to sign the written informed consent or was unable to complete the questionnaire.
Of 7281 pregnant women, 21 patients who were diagnosed with hypertension and diabetes before pregnancy or had blood glucose ≥ 7.0 mmol/L during early pregnancy and two pregnant women with gestational age ≥ 42 were excluded. 374 women were missing birth outcomes, 1696 women were missing interim sleep data, and 1282 women were missing covariate data. Finally, 5412 pregnant women were included in the analysis. A total of 5412 pregnant women were invited to complete a structured questionnaire at ≤15 weeks (first trimester) and 24-28 weeks (second trimester) of pregnancy.
Assessment of sleep duration
(a) During the first trimester and the second trimester, PSQI29 was used to assess the quality of sleep during the previous week. PSQI >5 was defined as poor sleep quality, and PSQI≤5 was defined as good sleep quality and was set as the control group. (b) Sleep duration of the past week in the first and second trimester of pregnancy was estimated: “What time did you go to bed at night in the past week?”, “How long does it usually take you to fall asleep each night?”, “What time do you usually get up in the morning?”. The sleep time was defined as the interval between the time to go to bed and the time to get up, which was divided into four grades based on the classification method of previous literature: ≤7 hours of nocturnal sleep is defined as short sleep time, 8-9 hours of nocturnal sleep is defined as adequate sleep time and control group 28, 10 hours of nocturnal sleep is defined as the length of long sleep, ≥11 hours of nocturnal sleep is defined as longer sleep time24,30. (c) Nap duration of the past week in the first and second trimester of pregnancy was estimated: ”How many minutes is the usual nap time for nearly a week?”. Nap duration was divided into five levels:0 minutes, 1-30 minutes, 31-60 minutes, 61-90 minutes, 90 minutes31. (d) Changes in sleep quality from the first trimester to the second trimester were divided into four mutually exclusive groups according to good/poor sleep quality during the first and second trimesters: always good (Sleep quality were all good in the first and second trimesters and were set as the control group), always poor (Both early and middle trimesters were poor), from good to poor, from poor to good. (e) Changes of sleep duration from the first trimester to the second trimester were divided into four mutually exclusive groups according to the groups of sleep duration during the first and second trimester: always good (Both the first and second trimesters of pregnancy were sufficient and were set as the control group), always poor (The first and second trimesters were short or long), from good to poor (From sufficient to short or long), from poor to good (From short or long to sufficient). (f) Changes in nap duration from the first trimester to the second trimester were divided into four mutually exclusive groups according to nap duration during the first and second trimester: always good (In the first and second trimesters of pregnancy, the napping duration was 1-60 minutes and were set as the control group), always poor (No napping or napping duration ≥90 minutes in the first and second trimesters of pregnancy), from good to poor (From 1-60 minutes to no nap or nap duration ≥90 minutes), from poor to good (From no nap or nap duration ≥90 minutes to 1-60 minutes).
Measurement of birth outcomes
Information on birth outcomes was collected either through medical records. Full term was defined as 37 to 41 weeks32, and PTB was defined as less than 37 weeks of gestation14; LBW is defined as birth weight <2500g, macrosomia defined as birth weight ≥4000g, and normal birth weight (NBW) defined as weight ≥2 500 g to <4 000g 33; SGA defines newborns whose birth weight is below the 10th percentile of the average weight of children for the same gestational age, and larger than gestational age (LGA) is defined as a newborn whose birth weight is above the 90th percentile of the average weight of children for the same gestational age 34.
Definition of variables
Pregnant women in the first trimester and the second trimester were interviewed by trained investigators to complete structured questionnaires on the maternal sociodemographic characteristics, lifestyle and health status.
Age was treated as three groups: <25, 25-29, ≥30. Employment was categorized as two groups: unemployed and employed. Average family income was categorized as two groups: 49,999 yuan ≤ and ≥50,000 yuan. Education was categorized as two groups: senior high school or lower and college or above. Place of residence was categorized as two groups: urban and rural area. Smoking was categorized as three groups: current, former and never. Drinking was categorized as three groups: current, former and never. Parity was categorized as: 0 and ≥1. The Chinese version of the Pregnancy Physical Activity Questionnaire (PPAQ) 35was used to calculate the past week physical activity energy expenditure (MET-H/week) in the second trimester of pregnancy, which has been validated among pregnant women in China36. The physical activity were classified as low, medium and high. Prepregnancy weight was self-reported by the women, and the weight of women in the second trimester was measured by a hospital body fat meter. Weight and standing height were measured with light clothes and no shoes. Prepregnancy BMI (kg/m2) was calculated using self-reported prepregnancy weight (kg) divided by height squared (m). Prepregnancy BMI was divided into four categories: underweight (<18.5 kg/m2), normal weight (18.5-23.9 kg/m2), overweight (24.0-27.9 kg/m2) and obese (≥28.0 kg/m2) 37. GWG was classified according to prepregnancy BMI and the normal values recommended by the Institute of Medicine (IOM) 38: Underweight 12.5 to 18.0 kg; Normal 11.5-16.0 kg; 7.0-11.5 kg overweight; Obesity 5.0-9.0 kg. GWG within the recommended range was defined as appropriate, below the recommended range was defined as insufficient, and above the recommended range was defined as excessive. Preeclampsia was divided into two groups: yes and no; HDP was divided into two groups: yes and no; GDM was divided into two groups: yes and no.