Births, already in decline in the first years of the 20th century, dropped through the years of WW1 bottoming out at one-third of the level at the start of the century in 1918. A mini baby boom is evident in 1920, the highest number of births in a single year in the record. The decline takes hold again until 1933.
Don't get taken in my the erratic variations during WW2. 1939 - 40, 1943 - 45 and 1949 all have large numbers of entries not yet in the FreeBMD database.