It depends on how often you want to pay for professional pics but every 3 months is usually good. So NB, 3 months, 6 months, 9 months, and 12 months. After that generally every year. If you can't do that often NB, 6 months, and 12 months is good.
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Ditto pp about NB, 3, 6, 9, and 12. DH and I plan on taking DS every month though, and just doing a few filler pictures. It's cool to see how much they change in a month's time.
With DD, we did a panel that has 4 photos - one each at 3, 6, 9 and 12 months. We'll be doing that again with DS. We're also going to do newborn and newborn/big sister pictures with DS. I didn't do newborn photos with DD and lived to regret it! This will be an instance where MORE was done for the second baby, not less, haha!
Re: What months to get professional pix?
This is what our photographer suggested we do.
We will be doing this as well.