A five-month-old baby died from rickets after his parents insisted on following a strict eating regime as part of their religion, the Old Bailey heard.
Nkosiyapha Kunene, 36, and his wife Virginia, 32, could face jail after admitting the manslaughter of their son Ndingeko.
Acute rickets causes the bones to soften because of a deficiency of vitamin D, phosphorus or calcium, caused by poor diet and a lack of sunshine.
It is believed Ndingeko's parents belong to the Seventh-day Adventist Church, whose members follow a lacto-ovo vegetarian diet that allows milk and eggs, but not animal flesh.
Judge Mr Justice Singh told the Zimbabwe-born couple he was 'considering all options'.
He said: "I do not think anyone should go away with any illusions about that.
"The seriousness of the offence of which the life of a baby is lost will be apparent to everyone so I must bear that in mind."
The case was adjourned and the pair, who are on conditional bail, will be sentenced on February 28.
At a previous hearing, prosecutor Richard Whittam QC said: "This is clearly a tragic case. Because of the diet [Virginia Kunene] adhered to, the child became unwell.
"There may have been insufficient support about the health of the child but there were warnings from the family.
"There came a time when it was obvious her son required medical assistance and there was a real possibility of death."