Four hundred youngsters and their families from across the UK and around the Commonwealth flocked to the prestigious National Review of Queen's Scouts at Windsor Castle.
The pregnant Duchess of Cambridge handed out Scouting honours in a light green Mulberry coat.
More top news
The study by the National Childbirth Trust and National Federation of Women's Institutes found a lack of staff left some women frightened.
Extracts released by 10 Downing Street suggest Mrs May will say the UK will not settle for a "half-in, half-out" policy.
Milder for most of us