Koze: Based on what I remember of the story background (when reading it once upon a time), it's a race to get pregnant: probably the first one to become pregnant gets to marry the protagonist (who presumably has wealth or corporation-related reasons to make him a more-than-usual desirable partner).
I confess, pregnant women physically racing /would/ be an interesting twist on that fairly-generic concept.
...I remember a thought experiment about women at various stages of pregnancy, put on treadmills that seemed to (but didn't actually) drop those who fell off the end into a pit of death... motivated by fear of death (of themselves and the foetus), the observer getting to observe all the stages and forms of exhaustion... horribly unethical if done in reality, of course.
I should also check what the title actually says, since I see there's a word missing...
Huh. Even the first word which is present should be 'Botsuraku' rather than 'Kanraku'. They admittedly mean similar things, but the first character is different.
...and the missing word is 'chijoku'. The title is then indeed fairly ambiguous, though pregnant women racing might be 'ninpu race' rather than 'ninshin race' (race to become pregnant). Hm.
actually the entire story is about the 2 girls who are well relatively poor to a degree (reality their families were also rich but lost in power) so their parents kinds forced them to an arranged marriage with the protagonist of the story for them to marry the protagonist whose family is filthy rich (IIRC his family's company bought out the companies of the heroines)... basically thats the gist of it