The impact of maternal smoking on embryonic morphological development: the Rotterdam Periconception Cohort.

study questionIs periconceptional maternal smoking associated with embryonic morphological development in ongoing pregnancies? SUMMARY ANSWER: Smoking during the periconceptional period is associated with a delayed embryonic morphological development which is not fully recuperated beyond the first trimester of pregnancy. WHAT IS KNOWN ALREADY: Smoking during pregnancy decreases prenatal growth, increasing the risk of preterm birth, small for gestational age (GA) and childhood obesity. STUDY DESIGN, SIZE, DURATION: Between 2010 and 2018, 689 women with ongoing singleton pregnancies were periconceptionally enrolled in a prospective cohort study with follow-up until 1 year after delivery. PARTICIPANTS/MATERIALS, SETTING,

methodsBetween 7 + 0 and 10 + 3 weeks, GA serial three-dimensional transvaginal ultrasound scans were performed. Embryonic morphological development as assessed by the Carnegie developmental stages was evaluated using Virtual Reality techniques. In the absence of fetal morphology classification methods beyond the embryonic period, fetal ultrasound measurements at around 20 weeks' GA, and birth weight were used to assess fetal growth. Linear mixed models were used to evaluate the association between smoking and the Carnegie stages. Regarding first-trimester morphological development, we additionally stratified our findings for mode of conception. Multiple linear regression models were used to study the association between smoking, fetal growth and birth weight. To investigate to which extent delayed embryonic morphological development mediated the effect of smoking, contemporary mediation analysis was used. Adjustments were made for potential confounders and other covariates. MAIN RESULTS AND THE ROLE OF CHANCE: A total of 689 singleton ongoing pregnancies were included and 1210 Carnegie stages were determined. Maternal periconceptional smoking represented by the number of cigarettes/day was associated with a slight non-significant delay of the Carnegie stages (βcigarettes/day = -0.058, 95% CI -0.122; 0.007, P = 0.080). Smoking of ≥10 cigarettes/day showed the strongest association (β≥10 cigarettes/day = -0.352, 95% CI -0.648; -0.057, P = 0.019), as reflected by a 0.9-day delay in reaching the final Carnegie stage. Stratification for mode of conception showed a stronger negative association between the number of cigarettes/day in the IVF/ICSI group (βcigarettes/day = -0.126, 95% CI -0.200; -0.051, P = 0.001) compared to naturally conceived pregnancies (βcigarettes/day = 0.009, 95% CI -0.093; 0.111, P = 0.867). In the IVF/ICSI group, periconceptional smoking of ≥10 cigarettes/day was associated with in a 1.6 day delay in reaching the final Carnegie stage (β≥10 cigarettes/day = -0.510, 95% CI -0.834; -0.186, P = 0.002). In the second trimester, periconceptional smoking was associated with a smaller femur length (βcigarettes/day = -0.077, 95% CI -0.147; -0.008, P = 0.029) and a larger head circumference (β1-9 cigarettes/day = 0.290, 95% CI 0.065; 0.514, P = 0.012). Smoking was associated with a lower birth weight, with a dose-response effect (βcigarettes/day = -0.150, 95% CI -0.233; -0.068, P < 0.001). Furthermore, using the unadjusted model, 40-60% of the association between smoking and fetal ultrasound parameters and 6.3% of the association between smoking and birth weight can be explained by a delayed embryonic morphology. LIMITATIONS, REASONS FOR CAUTION: The study population was recruited from a tertiary referral center. Smoking habits were explored using self-reported questionnaires and checked for consistency by trained researchers. WIDER IMPLICATIONS OF THE

findingsThis study shows that the association of periconceptional maternal smoking and human morphological development can already be detected early in the first trimester of pregnancy using embryonic morphology as outcome. One of the key messages of this study is that the delay, or dysregulation, in embryonic morphology is associated with allometric growth reflected by smaller fetal measurements at 20 weeks gestation and lower weight at birth. The delay in embryonic morphology, measured in early pregnancy, cannot be recuperated during the pregnancy. The results of this study emphasize the importance of smoking intervention programs prior to conception. More research is warranted to assess the association between periconceptional smoking cessation and embryonic development. STUDY FUNDING/COMPETING INTEREST(S): The work was funded by the Department of Obstetrics and Gynaecology, Erasmus MC, University Medical Centre, Rotterdam, The Netherlands. The authors declare no conflicts of interest. TRIAL REGISTRATION NUMBER: N/A.