Marshall and his mother, Rachael Hedges, are the last participants to be recruited in the 807-infant survey.
It is organised through University of Otago's department of women's and children's health and backed by a $2.1 million grant from the Health Research Council.
The district health board has also provided funding.
Miss Hedges said it was "pretty cool" that Marshall, who was born last Friday, weighing 3kg (6 pounds, 11 ounces), was part of the study.
He was in the observation (or control) group and she was happy to be involved in a study which could help provide significant health benefits.
The prevention of overweight in infancy (Poi) study is a three-year randomised controlled trial.
It is designed to assess whether increasing parental support with early feeding, activity and sleep reduces the rate of excessive weight gain during infancy and overweight status at the age of 3.
Prof Barry Taylor, who heads the university department, said study participants had been recruited over the past 18 months and organisers were "over the moon" that a "major milestone" had been reached.
Study organisers said there was "convincing evidence that growth in early childhood set the pattern for future growth later in childhood and, if excessive, childhood and adult obesity".
About a third of children aged 5-14 years were classified as overweight or obese.
Many observational studies had linked less sleep with obesity.
"Sleep difficulties are found in between a quarter and a third of all infants and very young children, and effective ways of managing these have been developed," organisers said.
The study has four intervention groups, each of about 200 infants and all receiving standard Well Child care support.
Within this, a control group was carefully measured but received no extra intervention.
A sleep group receives extra sessions aimed at increasing infant sleep duration, while improving parental wellbeing.
Another group receives support to improve diet and activity, and a further group, funded by the health board, receives both the sleep and the food and activity interventions.
Prof Taylor was also one of three principal researchers in a successful study of cot deaths (1987-89) which clarified key risk factors in cot death.
Subsequent interventions, including ensuring infants slept on their backs, had helped greatly reduce the New Zealand cot death toll.
Prof Taylor said concern was growing about a future obesity epidemic in New Zealand and its adverse effect on health, including through increased diabetes and heart disease.
The cot death study and the ensuing drop in death rate highlighted the value of research which clarified risk factors and provided an evidence-based assessment of health interventions, he said.
