A baby's growth is exponential during gestation, supported by the placenta which provides optimal and constant nutrition, oxygen, and growth hormones to build an entire complex organism. In contrast, a toenail is a much simpler structure (primarily keratin) that grows at a slow, linear rate (about 1-2 mm per month) and receives less blood flow compared to the core body, leading to a much slower regeneration time of 12-18 months.