Java mouse-deer are capable of breeding at any time during the year, and this has been observed during captivity. However, some sources have observed that the breeding season for the Java mouse-deer in nature occurs from November to December. Additionally, female mouse-deer have the potential to be pregnant throughout most of their adult life, and they are capable of conceiving 85–155 minutes after giving birth. The Java mouse-deer's gestation period usually lasts 4.5 months, or 144 days. Typical litters consist of a single fawn, which resembles a miniature adult, although the tusk-like incisors prevalent in males are not visible in the young mouse-deer. The average mass of a newborn fawn is , and these precocial young are capable of standing within 30 minutes after birth. Fawns are capable of eating solid food within two weeks, yet it takes around 12 weeks to completely wean the fawns. On average, it takes the young, both male and female, 167 days (~5 months) to reach sexual maturity. Mouse-deer have been observed to live up to 14 years in captivity, but their lifespan in nature is still an open question.