Studies consistently show a relationship between social disadvantage and low birthweight. Many countries have programmes offering special assistance to women thought to be at risk for giving birth to a low birthweight infant. These programmes, collectively referred to in this review as additional social support, may include emotional support, which gives a person a feeling of being loved and cared for, tangible/instrumental support, in the form of direct assistance/home visits, and informational support, through the provision of advice, guidance and counselling. The programmes may be delivered by multidisciplinary teams of health professionals, specially trained lay workers, or a combination of lay and professional workers. This is an update of a review first published in 2003 and updated in 2010. Objectives The primary objective was to assess the effects of programmes offering additional social support (emotional, instrumental/tangible and informational) compared with routine care, for pregnant women believed to be at high risk for giving birth to babies that are either preterm (less than 37 weeks' gestation) or weigh less than 2500 g, or both, at birth. Secondary objectives were to determine whether the effectiveness of support was mediated by timing of onset (early versus later in pregnancy) or type of provider (healthcare professional or lay person). Search methods For this update, we searched Cochrane Pregnancy and Childbirth's Trials Register, ClinicalTrials.gov, the WHO International Clinical Trials Registry Platform (ICTRP) on 5 February 2018, and reference lists of retrieved studies. Selection criteria Randomised trials of additional social support during at-risk pregnancy by either a professional (social worker, midwife, or nurse) or specially trained lay person, compared to routine care. We defined additional social support as some form of emotional support (e.g. caring, empathy, trust), tangible/instrumental support (e.g. transportation to clinic appointments, home visits complemented with phone calls, help with household responsibilities) or informational support (advice and counselling about nutrition, rest, stress management, use of alcohol/recreational drugs).