Smoking by mothers in the period around conception is linked to a delay in embryonic development, small foetuses at the 20-week ultrasound and lower birth weight.
Embryonic development is the different stages in the development of an embryo. The study is the first study to investigate the association between smoking by mothers from 14 weeks before conception, up to 10 weeks following conception and embryonic development. The researchers assessed the external and internal shape of the embryo (the morphology) during ongoing pregnancies. The team of researchers utilised virtual reality technology to monitor embryonic development and they compared the morphology against established stages of embryo development, known as the Carnegie Stages.
The study is published in Human Reproduction.
Following embryonic development
The study followed 689 women with singleton pregnancies between 2010 and 2018. The mothers were taking part in the ongoing Rotterdam periconceptional cohort, a large prospective study embedded in inpatient care in the department of obstetrics and gynaecology at Erasmus MC, University Medical Center, Rotterdam, which is a tertiary care hospital for preconception and prenatal care.
The researchers compared the morphology against the established stages of embryonic development, known as the Carnegie Stages. The Carnegie Stages only covers embryonic development for the first ten weeks of gestation; therefore, the researchers could not compare the embryo shapes against an agreed standard beyond this stage, but the ultrasound scans and birth weights provided developmental information instead, including head and abdominal circumferences and thigh bone length.
The study utilised new imaging techniques, including virtual reality, to gain a deeper insight into embryonic development in greater detail, rather than rely on information gained from laboratory research.
What did the researchers find?
The researchers discovered that by the tenth week of pregnancy, embryonic development was delayed by nearly one day in women who smoked ten or more cigarettes a day compared to non-smokers and by 1.6 days in smokers who conceived by in vitro fertilisation (IVF) and intracytoplasmic sperm injection (ICSI).
Dr Melek Rousian, a gynaecologist at Erasmus MC, University Medical Center, Rotterdam, The Netherlands, who led the study, said: “One of the key messages of this study is that the delay in embryonic development due to mothers smoking in the periconceptional period is also associated with smaller foetal measurements at the 20-week ultrasound scan and lower birth weight. Part of foetal and neonatal outcomes can be explained by smoking during the periconceptional period and the delay in embryonic development.”
Furthermore, the embryos were unable to reach the standard level of embryonic development throughout the pregnancy. The researchers also found that the babies were more likely to be born small for gestational age and with a median (average) birth weight of 93 grams lower than babies born to non-smoking women.
“The impact of periconceptional maternal smoking on delaying embryonic development appears to have a greater effect in the second trimester of pregnancy than at birth,” said Dr Rousian. “We think that perhaps there is some catch-up growth during the second and third trimesters, but the delay in morphological development cannot be fully recuperated during the course of the pregnancy, as is shown by the 20-week ultrasound scans and birth weights.”
“The results of this study emphasise the importance of smoking cessation prior to conception and that efforts to help women stop smoking should focus on this time window. If possible, women should stop smoking from the very moment they plan to become pregnant, but it’s always a good thing to stop smoking anyway, particularly at any stage of pregnancy. Smoking not only impacts an embryo’s growth during pregnancy and birth weight but also embryo development right from the very early stages of pregnancy,” she said.
“We were able to show a dose-response effect of maternal smoking on embryonic morphology and foetal growth; the more cigarettes a woman smoked, the greater the developmental delay. This stresses the importance of public health initiatives to promote preconception education and care, including effective programmes to assist couples to stop smoking.”
