You should actually take into account the 9 months preceding the birth, so if you think the next baby will come somewhere by the end 2020, maybe you should target something in the very beginning of 2020. Talking from experience here - I have my Ironman next Sunday and the next baby is expected for end of August.
This was all planned, so we were OK on that, but each pregnancy is different and this one proved to be much more difficult for the mom. She is helping me as much as she can and she did a great job, but I quite often felt guilty for spending so much time on training instead of taking care of her and kids (we have 2 already).
Also, I felt like a high speed robot sometimes - running back 15km from work instead to save time on commute, then straight into kitchen, make dinner for kids, prepare food for next day, put them to sleep, then only 2 hours later get a shower… This was actually harder than the training itself (but still loved it 
)