Frederik and Ingrid married in March 1935 and Queen Margrethe was not born until April 1940. In the days before really reliable contraception and careers for wives it was entirely usual for the first child to arrive within a year of the wedding (I did in 1959 - a very bare nine months!), so a gap of five years does suggest problems, especially as Frederik was heir apparent. I note that the current Crown Prince arrived just over 11 months after his parents' wedding and Prince Joachim barely a year later. Among British royalty Prince Charles arrived a week short of his parents' first wedding anniversary, and Prince William also 11 months after the wedding. Going back a little, Queen Victoria's first daughter arrived ten months after the wedding and the future Edward VII 11 months after that. Afterwardss, Victoria and Albert slowed down their rate of procreation somewhat.