During a pregnancy, the endometrium, or lining of the uterus, is referred to as the decidua in mammals. This portion of the uterine lining begins to form as soon as implantation of the embryo occurs, and it plays a major role in the formation of the placenta. Among other things, it helps nourish the embryo before the placenta is fully formed.
Roughly a week after ovulation, the uterine lining beings to thicken and form more blood vessels. This is in preparation for a possible embryo. If there is no pregnancy, this lining is shed roughly every month during something commonly known as a period.
If a female's egg is fertilized, it will usually travel to the uterus, where it implants into the uterine wall. This action triggers something that is known as the decidual reaction. During this, the topmost layer of the uterine lining thickens even more. In the area that the embryo implants itself to the uterus, the decidua grows around it an engulfs the embryo. This is known as the decidua capsularis.
There are two other main parts of the decidua. The area between the embryo and the uterine wall is known as the decidua basalis. All other parts of the decidua in the uterus are known as the decidua pariatalis.
Until the complete formation of the placenta, this lining is the main source of nutrition for the embryo. Like the placenta, it also allows waste materials to pass out of the embryo. It also protects the embryo and ensures that it is not destroyed by the mother's immune system. Female hormones and growth hormones necessary for a healthy pregnancy are also released from the decidua.
In the event of a miscarriage, the decidua is shed, and leaves the body along with the embryo, similar to a menstrual period. Sometimes, however, the decidua can be shed without the embryo. If this happens, there can be a number of complications and health problems for the mother.
An ectopic pregnancy is also referred to as a tubal or extrauterine pregnancy. During an ectopic pregnancy, the embryo does not implant into the uterine lining, but into another part of the female anatomy. In the majority of ectopic pregnancies, the embryo implants itself into the fallopian tubes, but in a few cases it can implant into the cervix or other part of the female anatomy.
Sometimes during an ectopic pregnancy, the decidua will pass out of the body, similar to a miscarriage. The embryo, however, will still be implanted into the wall of the fallopian tube. If left unattended, this can cause the organ to swell and rupture, resulting in severe bleeding and, possibly, death of the mother.