BACKGROUND: The 2016 World Health Organization Antenatal Guidelines and the 2015 South African Maternal and Child Health Guidelines recommend one early antenatal ultrasound scan to establish gestational age and to detect multiple pregnancies and fetal abnormalities. Prior research indicates that ultrasound scan can also increase parental-fetal attachment. We aim to establish whether, compared to routine care, messages to promote parental attachment and healthy child development, conducted during one or two pregnancy ultrasound scans, improve early child development and growth, exclusive breastfeeding, parental-child interactions and prenatal and postnatal clinic attendance. METHODS: The effect of messages to sensitise mothers and fathers to fetal development will be tested in a three-armed randomised trial with 100 mothers and their partners from Soweto, Johannesburg in each arm. The primary outcome is child development at 6 months postnatally. Secondary outcomes include infant feeding, parental attachment and interaction, parental mental health and infant growth, assessed at 6 weeks and 6 months. Parents in Arm 1 receive a fetal ultrasound scan < 25 weeks during routine antenatal care at tertiary hospitals, and a second standard ultrasound scan at the research site within 2 weeks. Arm 2 participants receive the routine antenatal ultrasound scan and an additional ultrasound scan < 25 weeks at the research site, together with messages to promote parental attachment and healthy child development. Arm 3 participants receive the routine ultrasound scan and two additional ultrasound scans at the research site, < 25 weeks and < 36 weeks, together with messages to promote parental attachment and healthy child development. DISCUSSION: Evidence from high-income countries suggests that first-time prospective mothers and fathers enjoy seeing their fetus during ultrasound scan and that it is an emotional experience. A number of studies have found that ultrasound scan increases maternal attachment during pregnancy, a predictor of positive parent-infant interactions which, in turn, promotes healthy infant development. It is generally agreed that studies are needed which follow up parental-child behaviour and healthy child development postnatally, include fathers and examine the construct in a wider diversity of settings, especially in low and middle-income countries. Testing the added benefits of pregnancy ultrasound scan for child development is a gap that the proposed trial in South Africa seeks to address. TRIAL REGISTRATION: Pan African Clinical Trials Registry, PACTR201808107241133. Registered on 15 August 2018.