The 33-year-old socialite, who's married to billionaire businessman James Rothschild, 32, wore a long white dress with black trim and patterns of cherries, accessorizing with a designer black purse and pointy red heels.
The 5ft7 heiress had her blonde locks parted and down, amplifying her complexion with blush.
It's been a month of milestones for Nicki, whose daughter Lily Grace Victoria Rothschild turned one on July 8, two days before the blonde Beauty celebrated her two-year-anniversary with Rothschild July 10.
And insiders told the DailyMail earlier this month that she's expecting her second baby, and is about three months along.
Fashionista: The famous heiress donned a cute ensemble for the midsummer night show