Animal reproduction can occur through sexual or asexual means. Sexual reproduction involves the fusion of male and female gametes to form a zygote, while asexual reproduction does not. Many invertebrates can reproduce asexually through fission, budding or parthenogenesis. Sexual reproduction is more common and involves hormonally regulated cycles and the production of gametes in ovaries and testes. Fertilization can be external or internal and development occurs over three trimesters culminating in birth.