The pressure for her to deliver a boy, a coming King, was enormous. So when she gave birth to four girls in a row, it wasn't easy for her.
Until the rules were changed to allow for Princess Victoria to become Queen in her own right, the need for a male heir was dominant. The pressure can be seen based on the following situation when Sybilla was married:
GUSTAF V (1858- 1950)
GUSTAF VI ADOLF (1882-1973
Gustaf Adolf Oscar Fredrik Arthur Edmund, Duke of Westerbotten 1906- 1947); m. 1932 Sibylle of Saxe-Coburg-Gotha (1908-1972)
Margaretha b.1934
Birgitta b.1937
Désirée b.1938
Christina b.1943
CARL XVI GUSTAF b.1946 (after 12 yrs marriage)
Sigvard, Duke of Uppland, renounced his titles and was created Count of Wisborg 1951 (1907- 2002); m.(1) 1934 (div 1943) Erica Patzek (2) 1943 (div 1961) Sonia de Robbert (3) 1961 Marianne Lindberg [So both his marriages had been unequal and he'd alrady been divorced once]
Ingrid (1910- 2000); m.1935 Frederik IX of Denmark (1899- 1972)
Bertil, Duke of Halland (1912- 1997); m. 1976 Lilian Davies (b. 1915)
Carl Johan, Duke of Dalecarlie, renounced titles and was created Count of Wisborg 1951 (b. 1916); m. (1) 1946 Kerstin Wijkmark (2) 1988 (div?) Countess Gunilla Wachtmeister af Johannishus [Married unequally shortly before Carl Gustav was born creating a lot of ill-feeling in the family since the pressure was mounting for a male heir]
That was Gustav Adolf's children. Then you go to his brothers:
William, Duke of Södermanland (1884- 1965); m. 1908 (div 1914) GDss Marie (1890- 1958)
Lennart, renounced his rights and was created Count of Wisborg 1951 (1909- 2004); m. (1) 1932 (div 1972) Karin Nissvandt (2) 1972 Sonja Haunz [He already contracted an unequal marriage]
Erik, Duke of Västmanland (1889- 1918) [never married]
Then going back to Gustav V’s brothers, his brother Oscar had renounced his rights to marry a commoner. Carl had married Ingeborg of Denmark and but only had one son (Carl Jr). Carl Jr, Duke of Östergötland, renounced his rights and was granted the title Prince Bernadotte by Albert I of Belgium 1937 (1911- 2003); m.(1 1937 (div 1951) Countess Elsa von Rosen (2) 1954 (div 1961) Ann Margareta (3) 1978 Kristine Rivelsrud. So he was ineligible.
So you can see for all the men in the family there weren’t a lot of heirs. It was somewhat reminiscent of George III and all his sons yet no heirs when Princess Charlotte died. Prince Bertil (if you look at his date of marriage) performed a great sacrifice. He was in love with Lilian (who was from Wales) but because of the dearth of heirs put off marrying (which would’ve meant renouncing his titles and thus becoming ineligible) until after his nephew was married. For this sacrifice, which prevented him from having children of his own, when he did marry, Lilian was granted the title of ‘Princess’ (which apparently annoyed his other siblings who’d had to renounce but oh well) and is still a beloved figure in the Swedish family and often seen at family events. Apparently she is seen as a grandmother figure to CPss Victoria, Prince Carl and Princess Madeleine.