BACKGROUND: Low birth weight has important short- and long-term health implications. Previously it has been shown that pregnancies affected by hyperemesis gravidarum in the mother are at higher risk of having low birth weight offspring. In this study we tested whether such risks are also evident with less severe nausea and vomiting in pregnancy. METHODS: One thousand two hundred thirty-eight women in the prospective Cambridge Baby Growth Study filled in pregnancy questionnaires which included questions relating to adverse effects of pregnancy and drugs taken during that time. Ordinal logistic regression models, adjusted for parity, ethnicity, marital and smoking status were used to relate the risk of giving birth to low birth weight (< 2.5 kg) babies to nausea and/or vomiting in pregnancy that were not treated with anti-emetics and did not report suffering from hyperemesis gravidarum. RESULTS: Only three women in the cohort reported having had hyperemesis gravidarum although a further 17 women reported taking anti-emetics during pregnancy. Of those 1218 women who did not take anti-emetics 286 (23.5%) did not experience nausea or vomiting, 467 (38.3%) experienced nausea but not vomiting and 465 experienced vomiting (38.2%). Vomiting during pregnancy was associated with higher risk of having a low birth weight baby (odds ratio 3.5 (1.2, 10.8), p = 0.03). The risk associated with vomiting was found in the first (p = 0.01) and second (p = 0.01) trimesters but not the third (p = 1.0). The higher risk was not evident in those women who only experienced nausea (odds ratio 1.0 (0.3, 4.0), p = 1.0). CONCLUSIONS: Vomiting in early pregnancy, even when not perceived to be sufficiently severe to merit treatment, is associated with a higher risk of delivering a low birth weight baby. Early pregnancy vomiting might therefore be usable as a marker of higher risk of low birth weight in pregnancy. This may be of benefit in situations where routine ultrasound is not available to distinguish prematurity from fetal growth restriction, so low birth weight is used as an alternative.