Hi Debbie,
As with so many processes, it's a range, rather than an event. For some it will be earlier or later. So I would never give a mother a figure, as that can cause anxiety. Suffice it to say that I haven't seen an instance where the milk didn't revert to colostrum by the time the new baby was born, even when the toddler was a very avid breastfeeder, and even (in my personal situation) where in an emergency situation I'd tripled my breastmilk output overnight in the early months of pregnancy. Once the baby is delivered, there will be colostrum, even if it hasn't been evident before.
Virginia

Dr Virginia Thorley, PhD, IBCLC, FILCA
Private Practice Lactation Consultant (first certified 1985), 
Brisbane, Queensland
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Debbie Dixon wrote:
Just wondering at what point does breastmilk revert back to colostrum during pregnancy (in the case of a mother who is breastfeeding an 18 month old & is 12 weeks pregnant)?  Is it a gradual transition throughout the pregnancy or does it happen closer to the time of birth of the new baby?  And how should the mother approach this as regards making sure the new baby receives the colostrum in the early days if the toddler is still feeding intermittently? 
Thank you for your information,Debbie Dixon IBCLCDubbo NSWAUSTRALIA      
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