The roe deer rut falls in July and August, months earlier than in most temperate deer. Gestation would then deliver fawns in late winter, into the worst possible conditions.
Instead the embryo stops. After fertilisation the blastocyst grows very slowly and remains unattached in the uterus for roughly four to five months, a state known as embryonic diapause or delayed implantation. Around December or January it implants and normal development resumes, so fawns arrive in May and June alongside the flush of new vegetation. Total elapsed time from mating to birth is about ten months, but only around five of those involve real growth.
Diapause is common in bears, mustelids and seals, and the roe deer is the only member of the deer family known to use it. The molecular signals that hold the blastocyst dormant and then release it are still being worked out, and photoperiod is the leading but not the only candidate trigger.