I asked my Dr about that when I was ~5 months pregnant with DD2. DD1 was at least 33 pounds then. She told me that there was no concern with regards to the pregnancy but that I could strain my back if I wasn't careful. I picked her up and carried her for short distances up until I was ~30 weeks and then I just told her that mama couldn't pick her up anymore. It was hard but my back and shoulders were getting really sore.
If your body is aching then it is probably time to stop picking the kids up.
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Mother to DD-A July 2008
and DD-B-November 2010