The Duchess of Cambridge could miss taking Prince George for his first day at school after suffering with severe morning sickness.
The Duke of Cambridge and Kate had been expected to drop off their four-year-old son as he starts at Thomas's Battersea, a fee-paying independent school in south London.
However, it is not yet clear if the Duchess, who is less than 12 weeks pregnant, will be well enough to join the school run on Thursday.
It is understood a decision over whether she will be present will be made in the morning.
George and his parents are due to be greeted by Helen Haslem, head of lower school, when they arrive at the school gates before being escorted to his reception classroom.
The prince - thought to be one of the youngest in his year - is expected to have a gentle introduction to what is likely to be a tiring first term at school.
Along with his classmates, he will spend the day getting to know teachers, adults and other children as well as completing the important task of finding his classroom peg.
George will be dressed in a smart new uniform, comprising a navy v-neck pullover, matching Bermuda-style shorts, long red socks and black shoes.
Kate was forced to pull out of two public engagements on Monday and Tuesday after suffering from hyperemesis gravidarum, as she did in her previous pregnancies.
On Tuesday - a day after it was announced they are expecting their third child - William said Kate was well but admitted: "There's not much sleep going on at the moment."
He said: "We need Catherine to get over this first bit and then we can start celebrating. It's always a bit anxious to start with, but she's well."
George's new school, where fees cost from £17,604 a year, has been described by the Good Schools Guide as: "A big, busy, slightly chaotic school for cosmopolitan parents who want their children to have the best English education money can buy.
"That is what they want and, to a large degree, that is what they get."
The young prince and his classmates will be taught a range of subjects and activities from ballet and art to drama, French, music and physical education.
Kate has already said she is not sure her son "has any idea what's going to hit him'' when he starts school.