You can start right away, we already sign to our son and our daughter signs to him as well. I personally would just start with the basic signs Milk, More, Please, All Done, Mom, Dad and maybe Thank you. Than as you want to expand their signs you can start to look for books. I eventually moved to actual ASL books (not sign language books)
I don't do it on a daily basis either. I'll just sign "all done" when we're done changing his diaper though, just without thinking about it. I also tried to do "more" and "please". I think the fewer you start with, the faster they learn them.
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I already sign with him, but it's obviously too early for him to sign back. I figure I might as well get in the habit myself. The book I have is "Teach Your baby To Sign" by Monica Beyer. I like it.
Sept 2013 started TTC#2 (never got PP period so hoping for the first egg to drop)
Jan 2014 - diagnosed POF (Premature Ovarian Failure) and told pregnancy not possible without egg donor because my ovaries would never function again with my hormone levels (FSH 136, LH 98.6, AMH <.03, estradiol 0, HCG 7) - using acupuncture, chiropractic care and TCM herbs / supplements
March 11, 2014 ovulated despite every doctors prognosis!!!
Oct 2014 hormones (FSH 48, Estradiol 89, HCG 1) not good, but heckuva lot better!
Nov. 16, 2015 FET of single adopted 5AA embryo - BFN
Re: BABY SIGN LANGUAGE