Sexual receptivity is not limited to a timeframe.
Estrous cycles are characteristic of most mammals:
The endometrium is reabsorbed by the uterus
Sexual receptivity is limited to a ďheatĒ period
The length and frequency of estrus cycles varies from species to species.
An egg develops into an embryo in a series of predictable events.
Conception = fertilization of an egg by a sperm, occurs in the oviduct.
The resulting zygote begins to divide by mitosis in a process called cleavage.
Division of cells gives rise to a blastocyst, a ball of cells with a cavity.
(a) From ovulation to implantation
(b) Implantation of blastocyst
The blastocyst implants
Inner cell mass
After blastocyst formation, the embryo implants into the endometrium.
The embryo releases human chorionic gonadotropin (hCG), which prevents menstruation.
Pregnancy, or gestation, is the condition of carrying one or more embryos in the uterus.
Duration of pregnancy in other species correlates with body size and maturity of the young at birth.
Pregnancies can terminate spontaneously due to chromosomal or developmental abnormalities.
An ectopic pregnancy occurs when a fertilized egg begins to develop in the fallopian tube.
Human gestation can be divided into three trimesters of about three months each.
The first trimester is the time of most radical change for both the mother and the embryo.
During implantation, the endometrium grows over the blastocyst.
During its first 2 to 4 weeks, the embryo obtains nutrients directly from the endometrium.
Meanwhile, the outer layer of the blastocyst, called the trophoblast, mingles with the endometrium and eventually forms the placenta.
Blood from the embryo travels to the placenta through arteries of the umbilical cord and returns via the umbilical vein.