Kim Kardashian West and Kanye West conceived their daughter the night she wore her first custom made Balmain gown.

Kim Kardashian West

Kim Kardashian West

Olivier Rousteing, the creative director at Balmain, has revealed the 'Keeping Up With The Kardashians' star was in her signature blue dress the night she got pregnant with North, now three.

Speaking in a video Q & A, he said: "You know what is the most amazing thing about this dress that - I don't know if I can say it but I'm going to say it. I'm sorry Kim ...

"It was worn the night when North was conceived. I don't know if I can say, maybe it's a secret, but I know that Kanye told me. So, I just wanted to share with you guys."

Meanwhile, the 31-year-old fashion designer described his first meeting with Kim - who also has Saint, 11 months, with husband Kanye - as "surprising, electric and lovely".

He shared: "My first meeting with Kim in three words - I would say surprising, I would say electric, and I would say love ... The first design that I sketch and I create for Kim was her dress for her Bachelorette party. This is a memory I will never never forget. She wanted something really unique and really special for a day that was special. So, I kind of create that Marie Antoinette costume and it was really really beautiful.

"I think she looked so great in that dress and I was really really happy with that because I love Kanye and I love Kim and altogether being part of that moment - fashion moment - I think it was really important to me. She's a unique woman and that dress was unique as well so it was really special to me."