BACKGROUND: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). DATA COLLECTION AND ANALYSIS:Two review authors independently assessed studies for inclusion and risk of bias, extracted data and checked them for accuracy. We assessed the quality of the evidence using the GRADE approach. MAIN RESULTS:This updated review includes a total of 25 studies, with outcome data for 11,246 mothers and babies enrolled in 21 studies. We assessed the overall risk of bias of included studies to be low or unclear, mainly because of limited reporting or uncertainty in how randomisation was generated or concealed (which led us to downgrade the quality of most outcomes to moderate), and the impracticability of blinding participants.When compared with routine care, programmes offering additional social support for at-risk pregnant women may slightly reduce the number of babies born with a birthweight less than 2500 g from 127 per 1000 to 120 per 1000 (risk ratio (RR) 0.94, 95% confidence interval (CI) 0.86 to 1.04; 16 studies, n = 11,770; moderate-quality evidence), and the number of babies born with a gestational age less than 37 weeks at birth from 128 per 1000 to 117 per 1000 (RR 0.92, 95% CI 0.84 to 1.01, 14 studies, n = 12,282; moderate-quality evidence), though the confidence intervals for the pooled effect for both of these outcomes just crossed the line of no effect, suggesting any effect is not large. There may be little or no difference between interventions for stillbirth/neonatal death (RR 1.11, 95% CI 0.88 to 1.41; 15 studies, n = 12,091; low-quality evidence). Secondary outcomes of moderate quality suggested that there is probably a reduction in caesarean section (from 215 per 1000 to 194 per 1000; RR 0.90, 95% CI 0.83 to 0.97; 15 studies, n = 9550), a reduction in the number of antenatal hospital admissions per participant (RR 0.78, 95% CI 0.68 to 0.91; 4 studies; n = 787), and a reduction in the mean number of hospitalisation episodes (mean difference -0.05, 95% CI -0.06 to -0.04; 1 study, n = 1525) in the social support group, compared to the controls.Postnatal depression and women's satisfaction were reported in different ways in the studies that considered these outcomes and so we could not include data in a meta-analysis. In one study postnatal depression appeared to be slightly lower in the support group in women who screened positively on the Edinbugh Postnatal Depression Scale at eight to 12 weeks postnatally (RR 0.74, 95% CI 0.55 to 1.01; 1 study, n = 1008; moderate-quality evidence). In another study, again postnatal depression appeared to be slightly lower in the support group and this was a self-report measure assessed at six weeks postnatally (RR 0.85, 95% CI 0.69 to 1.05; 1 study, n = 458; low-quality evidence). A higher proportion of women in one study reported that their prenatal care was very helpful in the supported group (RR 1.17, 95% CI 1.05 to 1.30; 1 study, n = 223; moderate-quality evidence), although in another study results were similar. Another study assessed satisfaction with prenatal care as being "not good" in 51 of 945 in the additional support group, compared with 45 of 942 in the usual care group.No studies considered long-term morbidity for the infant. No single outcome was reported in all studies. Subgroup analysis demonstrated consistency of effect when the support was provided by a healthcare professional or a trained lay worker.The descriptions of the additional social support were generally consistent across all studies and included emotional support, tangible support such as home visits, and informational support. AUTHORS' CONCLUSIONS:Pregnant women need the support of caring family members, friends, and health professionals. While programmes that offer additional social support during pregnancy are unlikely to have a large impact on the proportion of low birthweight babies or birth before 37 weeks' gestation and no impact on stillbirth or neonatal death, they may be helpful in reducing the likelihood of caesarean birth and antenatal hospital admission.