Years ago, I read in a magazine that there is no perfect time to have kids. If you are 80% sure you want them, go ahead because you won't get much closer to that 100%. At the time I thought that was a bit silly, but it turned out to be true. And I found myself applying that 80% rule to many other decisions in my life. While I might wonder how things could have turned out differently, I have not regretted any of the decisions I made. I'm not an impulsive person, I ponder quite a bit. But one can ponder forever and not accomplish anything...
I got married while in grad school and, as most relationships go, it wasn't really planned. It felt right enough for me to blurt out one day after dinner that I thought we should get married. The answer I got was 'OK, I'll call your bluff' and a beautiful ring that I wouldn't have done any better picking out myself. I wasn't bluffing, but I wasn't 100% sure either. I was anticipating a long engagement, because I wasn't sure I could take the time and plan the "big fat Brazilian wedding" I wanted. We ended up having 3 weddings, and by the end of the year my husband said he was done getting married, for life.
All that wedding stuff delayed my PhD defense for about 6 months, as I wasn't quite done. I realized later that I would never be. I could keep planning experiments my whole life. But one needs papers and I had to write things up. So I did, when I thought it was 80% there. Then I added the few extra experiments the reviewers suggested, and I still thought it was only 80%. I'm writing 2 papers now, and for both I have about 80% of the data. My contract has ended and I have only a few weeks before I move on. I wish I had an extra 6 months, but I'm sure I would think it was only 80% complete then too.
I believe I also used the 80% rule when picking my post docs. The labs seemed to be the right place for me to learn what I wanted. Were they my best choice? They could have been better, but I don't regret either of them. And both mentors were very understanding when I announced my pregnancies. I don't regret those at all, and the timing seems to have worked out well. I was told by a pregnant post doc during grad school that post docs were the best time to have kids. I took her advice, but I don't think there is a best time. I can feel the setback, but I don't think any other time would have been better. Either you want them or you don't. And if you do, just have them when you are 80% sure.