Tobacco smoking in pregnancy is now widely accepted as having adverse health outcomes for both the mother and fetus. Tobacco smoking in pregnancy is associated with increased incidence of miscarriage, stillbirth and preterm birth as well as low birthweight, respiratory infections, wheeze and asthma in childhood. E-cigarettes are a popular method for trying to quit smoking in Ireland and there has been an explosion in the use of e-cigarettes over the past ten years. However, there is currently insufficient evidence on their long-term safety and effectiveness as a smoking cessation tool. E-cigarettes contain varying combinations of compounds and flavours which are used differently in different e-cigarette types, with unknown long-term effects. Research has shown that pregnant women perceive e-cigarettes to be a healthier option when compared with tobacco smoking. But, there is very little known about the long-term health impact of exposure of unborn babies to e-cigarettes during pregnancy. The ECHO study will determine what the long-term health outcomes are in children born to mothers who use e-cigarettes during pregnancy. Specifically, we will focus on birth, nutritional, brain development and respiratory outcomes in children. To answer this research question, the ECHO study will recruit infants born to women who use e-cigarettes during pregnancy across three maternity hospitals and follow them up over 2 years. We will invite women at their booking visit to take part in this research study. We plan to also recruit a similar number of both non-smoking and tobacco smoking pregnant women for comparison. We will record a detailed record of e-cigarette and tobacco use by women during pregnancy as well as checking smoking using special monitoring tools. After the baby is born, we will perform growth measurements, neurocognitive assessments and a respiratory questionnaire at 6 months, one year and two years of age.