The couple, who tied the knot earlier this year, are already proud parents to 19-month-old son Sacha.
Tatiana is said to be suffering from morning sickness, which began in the early stages of her pregnancy.
Tatiana Santo Domingo and Andrea Casiraghi are already parents to baby son Sacha
Less than two weeks ago, Tatiana was pictured in London, where she attended a handbag launch dinner for Charlotte Olympia.
The fashion designer, who is one half of ethical clothing and accessories brand Muzungu Sisters, certainly had a glow about her as she was spotted mingling with other big names in the fashion industry.
Donning a black jacket with a quirky patterned lining, the Bohemian beauty looked as on-trend as ever, concealing her growing baby bump in the loose-fitting top. Tatiana left her brunette locks tumbling over her shoulders and wore a pop of red lipstick.
Tatiana Santo Domingo kept her growing baby bump under wraps at a recent London event
A couple of months ago, the New York-born beauty attended the wedding of Hugo Wilson and Maria Theresia Princess von Thurn und Taxis in Germany.
Tatiana, looking radiant in a purple, oversized shift dress, was accompanied by her husband Andrea, who wore a charcoal suit.
Tatiana Santo Domingo and Andrea Casiraghi married in February this year
Andrea and Tatiana themselves celebrated their own nuptials in Gstaad, Switzerland on 1 February. The couple chose the romantic eleventh century Rougemont Church to exchange their vows in a candlelit, Roman Catholic service.
In the presence of their family and friends – including Princess Caroline, Prince Albert and the groom's siblings Charlotte Casiraghi, Pierre Casiraghi and Princess Alexandra of Hanover – Andrea and Tatiana celebrated their second wedding.
The couple had already held a civil wedding service in Monaco in August 2013 – just five months after Tatiana had given birth to their baby boy Sacha.