In domestic dogs, sexual maturity begins to happen around age six to twelve months for both males and females,[3][155] although this can be delayed until up to two years old for some large breeds. This is the time at which female dogs will have their first estrous cycle. They will experience subsequent estrous cycles biannually, during which the body prepares for pregnancy. At the peak of the cycle, females will come into estrus, being mentally and physically receptive to copulation.[3] Because the ova survive and are capable of being fertilized for a week after ovulation, it is possible for a female to mate with more than one male.[3] Dogs bear their litters roughly 56 to 72 days after fertilization,[3][156] with an average of 63 days, although the length of gestation can vary. An average litter consists of about six puppies,[157] though this number may vary widely based on the breed of dog. In general, toy dogs produce from one to four puppies in each litter, while much larger breeds may average as many as twelve. Some dog breeds have acquired traits through selective breeding that interfere with reproduction. Male French Bulldogs, for instance, are incapable of mounting the female. For many dogs of this breed, the female must be artificially inseminated in order to reproduce. Canine reproduction is the process of sexual reproduction in domestic dogs. The anatomy of female dogs is similar to that of many other mammals. They have two ovaries located caudal to the kidneys in the abdominal cavity. They have a bicornuate (or two-horned) uterus which ends as the cervix, a short canal which connects to the vagina. The cervix is muscular with fibrous tissue support, and its opening closes in a stellate pattern. The vagina and urethra opens into the vestibule. The clitoris lies under the hood inside the vulva. The hood extends down and forms the outer boundaries of the vestibule as the labia minora. Bitches often have a vaginal stricture or hymen, which is a remnant of where the vagina and vestibule fused together during embryonic development. This structure is often asymptomatic and generally is not complete leaving an opening from the vagina to the vestibule. It is forced open further during mating. In domesticated species one of the first and strongest effects seen from selective breeding is selection for cooperation with the breeding process as directed by humans. One of the behaviors noted is the abolition of the pair bond seen in wild canines. The ability of the female domestic dog to come into estrus at any time of the year and usually twice a year is also valued. The amount of time between cycles varies greatly among individual dogs, but a particular dog's cycle tends to be consistent through her life. Conversely, wild species generally experience estrus on e a year, typically in late winter. Most dogs come into season for the first time between 6 and 12 months, although some larger breeds delay until as late as 2 years. Like most mammals, the age that a female first comes into season has multifocal contributions of genetic, hormonal, dietary and nutritional, and behavioral signals to both physical and sexual maturation. gonadal sex steroids, growth hormone, insulin-like growth factor (IGF-1) and leptin constitute prime determinants of evolving body composition and the onset of puberty[citation needed]. The relationship between body fat and body mass correlates well with the onset of puberty in most mammals.[citation needed] They then experience estrus about every seven months until old age. Female dogs do not experience menopause, although their cycles will become irregular and fertility becomes unpredictable as they become older. Dogs over around 7 or 8 years are not usually considered appropriate for breeding, but can still remain fertile.Female cycle A female dog's vulva in heat The average length of the reproductive cycle for females is 6 months. Females reach sexual maturity (puberty) between 8 and 18 months of age. There is a tremendous variability in the maturation age between breeds, and even within a breed of dog. The first stage of the reproductive cycle is proestrus, in which eggs in the ovaries begin to mature and estrogen levels begin to rise. During this stage males are attracted to non-receptive females. Initially, the vulva lips will swell up and become pliable, there are small amounts of bloody vaginal discharge, and signs of frequent urination and restlessness. Proestrus generally lasts 9 days. Estrus is the next stage, in which estrogen levels are high, mature eggs are released from ovaries, and the females mentally and physically become receptive to copulation. It is only during estrus that copulation will result in pregnancy. During proestrus and estrus, females may have a clear to bloody discharge. This stage is also known as "heat." The length of these cycles varies greatly between individuals. Proestrus and estrus can last anywhere from 5 days to 21 days. Diestrus is the period following mating. Diestrus lasts approximately 56 to 58 days in a pregnant female, and 60 to 100 days in a non-pregnant female. During both of these periods, progesterone levels are high. Because the hormonal profile of a pregnant female and a female in diestrus are the same, sometimes a non-pregnant female will go through a period of pseudo-pregnancy. At that time she may gain weight, have mammary gland development, produce milk, and exhibit nesting behaviours. Anestrus is the period of reproductive quiescence. The female has no attraction to or from the male. Anestrus generally lasts four to five months.