Humans are menstruating species, they have a menstrual cycle. Oestrogen, a hormone produced primarily by the ovaries, drives proliferation of cells in the lining of the womb. By day 14 of the menstrual cycle, an egg is released, this is the most fertile point of the cycle. So if you want to get pregnant, that is the perfect time. If the egg gets fertilised, it can be implanted into the womb and pregnancy can begin. If this happens out of sync. you will not get a pregnancy. The control of oestrogen and progesterone during the menstrual cycle happens under the control of the gonadotrophins (more hormones) which are secreted from the brain. Examples of gonadotrophin are luteinising hormone (LH) and follicle stimulating hormone (FSH). They control steroid synthesis in the ovaries and the release of the egg itself. The egg needs to be released at the right time to be implanted in the uterus (uterine lining). The words uterus and womb can be used interchangeably. When the egg starts to divide it becomes a blastocyst. The blastocyst will implant into the side of the uterus, if this is successful the uterus accepts the implantation (it biosenses the quality of the embryo). This is not a given in humans, the implantation signal comes from the outer cell layer of the egg, this signal is human chorionic gonadotrophin (hcG). In a pregnancy test, the hormone that is tested for is hcG because it is produced when we have a successful conception. Having hcG is the babies way of hijacking the maternal system because it acts similarly to LH and FSH. It acts upon the ovaries to maintain production of progesterone, so that it doesn’t enter the menstrual cycle. If you enter the menstruation cycle again, the whole lining including the egg will be removed, the lining is removed when we have a period. This happens until a placenta is established, once this happens the endocrine system can take over the whole process.
Even at an early stages of a pregnancy things can go wrong. A miscarriage occurs when the lining of the womb gone away. Pre-eclampsia can be a very serious, life threatening condition. Symptoms can be swelling, pain in the ribs, severe headache, high blood pressure and proteins found in the urine.
With pregnancy tests, there is the possibility for an oversensitive test, this is not necessarily a good thing. We don’t want to have people thinking they are pregnant and find out they or not. Additionally we do not want people to think they are pregnant then they have a miscarriage because of incorrect implantation.
After 8-12 weeks you get the luteal placental shift, this is where the placenta has now established itself and can take over the production of progesterone. So you no longer need the ovary to produce progesterone. Progesterone levels increase massively during pregnancy. Nuclear Progesterone receptor signalling is essential for pregnancy in mammals, without it the pregnancy will terminate. The abortion pill consists of a nuclear progesterone receptor antagonist.