This is such a fascinating topic, I was just reading up on it the other day! Legally we can't find out in India of course, but it's so exciting to think about. Apparently, the gender is decided at the moment of conception itself, even though we can't tell!
Biologically, it's decided right at conception when the egg and sperm meet. Because of my PCOS, I was just praying for a healthy pregnancy rather than thinking about these things. Focus on your health and development, every milestone is beautiful.
The baby's gender is fixed at conception itself, though the actual physical parts develop around week 11-12. In India, gender determination is strictly illegal, so just focus on keeping your weight and nutrition on track. Enjoy the surprise at birth, it's the best feeling!
I've heard it's decided on day one itself! Honestly, right now my baby's only gender seems to be 'hungry' because I am craving spicy pani puri 24/7. Whatever it is, as long as they let me eat in peace, I am happy!
The genetic blueprint is set at fertilization itself, so the deal is done on day one! Your baby is already a boy or girl, they are just taking their sweet time to grow. Don't worry about the gender, just prepare yourself for the sleepless nights ahead!