My experience with this issue:
I wasn't ready to have kids in my 20s. My husband was, but understood my career goals were important, so eventually stopped nagging me about it
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I matriculated in med school in 2003, and soon decided that it was time for us to have a child, and that I wanted one born just after 2nd year was done. I was 30 at the time, and everything worked out perfectly; my daughter was born in June of 2005. I ended up taking a year off afterwards, as my originally planned 6 months just wasn't enough time.
Due to a childcare hiccup, I took 2 rotations off in third year, knowing that I would need to delay graduation again if I wanted any time off during 4th year. It was valuable time with my daughter, as I afterwards found that I was finally able to come to terms with being away from her for extended periods of time. Some people can do that without flinching, but I surprisingly hated every second of separation during the beginning of 3rd year. That's something to consider- I never thought that would happen to me, but it did, and rather threw me for a loop.
I often think it would have been easier if I had been ready to have kids earlier, and they were pretty much independent people (ie school-aged) by the time I started med school. Or the other option, that I hadn't pursued other interests before college, and had gone the traditional route and had kids after residency (and had endured the back-in-the-day 110 hr weeks of an ortho resident like I originally planned!
) But that wasn't my life course, so whatever.
Last year, I decided we had to have another child before I started residency, because I am planning on doing surgery and realized I would be in my 40s after fellowship. We're due this summer with #2; I'll be taking another year off and decelerating my leftover credits to cover the whole year. The timing really works out well, other than the fact that it is very weird to be skipping through graduating classes and to know 10 times as many residents as I do students.
A big reason for this path for #2 is that I won't be asked to pay loans back until after my graduation date, so taking time off now instead of later makes financial sense. I will have the option of doing some research during that time if I so desire- helpful for competitive residencies. Also, I will be able to do more electives during my final year, as there is no cap, just an extended time during which to complete the minimums.
Most of all, I know, not even considering my age, that I do NOT want to have kids during residency. I've seen the horrible emotional struggles some of our residents have gone through when they try to return after a 1 or 2 month leave. I am strongly attached to being the primary caretaker during infancy, so that scenario just doesn't work for me. As difficult as it is to realize that it will have taken me 7 years to complete medical school, I would regret it so much more if I did not take advantage of this time with my children.
So, final analysis: If you REALLY feel emotionally- individually and as a couple- and financially ready to have kids early, go for it. Having a good support system once you are in med school is key. My support system is amazing (very involved and supportive husband, my mom living with us to do childcare- and cook and do laundry!- and lots of mommy friends for advice and fantastic hand-me-downs); I can't imagine being able to have all the options that I do without it.
If before med school isn't an option and you feel that after residency is too late- and I beg to differ that 30s is too late unless you already have risk factors for fertility issues- then during is a great time. I agree that first and second year are a great time. For me personally, they were 2 of the cruisest years of my entire educational life. I thought I would be the type at the library for hours every day, but it turned out that was entirely unnecessary for me- and I even did decently grade-wise and quite well on step 1.
Finally, despite all the declining fertility talk, I have had many friends have children in their 30s. The only ones who had any intervention were artificially inseminated due to lack of a male partner (either as single parents by choice or as lesbians). I have huge risk factors for fertility issues, but didn't have any trouble either. I was 31 when #1 was born, and will be 34 for #2; dh is 5 years older (TMI time here: both were conceived on the first try). As for birth control causing reduced fertility- I'd do some research on the validity of that fear. Personally I've never heard of that, but then I've never researched it.