Zed Nelson: 'I began this project in the summer of 1991. The wife of a friend was nine months pregnant, and I had an idea – based on time-lapse photography – to photograph them together as a couple, then soon after the birth, and then on the same day every year. I planned the photography sessions in a formal, almost scientific way. Each year the picture was made on the same date, against the same backdrop, under the same lighting. Now, nearly 20 years into the project it is beginning to get really interesting. While a boy grows before your very eyes, his parents change in more subtle ways. The body language fascinates me, between the growing boy and his parents. At first the son stays close to his mother, then he gains independence, and then increasingly bonds with and even mimics his father. These aren't quirks of the photographic moment, but cycles of the aging process, clearly played out in the contact sheets."

Photograph: Zed Nelson/INSTITUTE