In Annelise Bistrup's biography on Queen Ingrid (1997) it is mentioned that Ingrid was bedridden already when she was 2 months pregnant with Benedikte in order to prevent a miscarriage. Either before or after Benedikte, Ingrid actually did lose an unborn child, as told by Queen Margrethe in another biography by Annelise Bistrup ("Margrethe") published in 2005. At one point during the German occupation Ingrid told her husband: "You can tell father-in-law that if they do this or that, I will leave my bed, and then he won't have his next grandchild."
This is also mentioned in Bistrup's biography from 2005 and in Jon Bloch Skipper's book "Tre søstre" from 2008.