I'm raising my kids bilingual and it's an uphill battle to maintain a second language when they are immersed in English everywhere. But I have a curious observation from my sample size of 2. When I was pregnant with my first, I only spoke English. Then I started speaking my other language to her after she was born. So when I was pregnant with my second, he would hear me speak both languages (one to his sister and the other to everybody else). To this day, he has a much stronger affinity for this second language than his sister does, is more likely to speak it without prompting, and is better at it than she was at this age (better at it than English even). It's hard to say if that's because he heard me speak it while I was pregnant, or because once born, he had 2 people to hear it from and speak it with, as opposed to her only having me at the beginning. But it is interesting.