When Twi announced her pregnancy to Tempest
 
At the time, they weren’t sure if they wanted to have a kid. Tempest was scared she would mess up, and Twilight was afraid the baby would grow up under pressure. And, around that time, Rarity had a miscarriage and her baby almost died. Even though they turned out fine, it still shook Twilight.  
Eventually, they decided to let things run their course, and Tempest was transformed into a stallion so they could conceive. The plan was to try to conceive for a bit, and let Tempest turn back into a mare. If Twilight was pregnant, they would have their answer. If not, then they would either wait, or not have children. Twilight discovered she was pregnant and told the news.
 
alright, next picture, I’ll reveal the first kiddie ;D
 
Twilight Sparkle and Tempest Shadow belongs to Hasbro