About 24 hours later, the resulting zygote begins dividing, a process called cleavage. Cleavage continues, with the embryo becoming a ball of cells by the time it reaches the uterus 3 to 4 days after fertilisation. By about 1 week after fertilisation, cleavage has produced an embryonic stage called the blastocyst, a sphere of cells containing a cavity. In a process that takes several more days for completion, the blastocyst implants into the endometrium.
The embryo secretes hormones that signal its presence and control the mother′s reproductive system. One embryonic hormone, human chorionic gonadotropin (HCG), acts like pituitary LH to maintain secretion of progesterone and oestrogens by the corpus luteum through the first few months of pregnancy. In the absence of this hormonal override, the decline in maternal LH due to inhibition of the pituitary would result in menstruation and loss of the embryo. Levels of HCG in the maternal blood are so high that some is excreted in the urine, where it can be detected in pregnancy tests.
First Trimester
Human gestation can be divided for convenience 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. Let′s take up our story where we left off, at implantation. The endometrium responds to implantation by growing over the blastocyst. Differentiation of the embryo′s body structures now begins.
During its first 2 to 4 weeks of development, the embryo obtains nutrients directly from the endometrium. Meanwhile, the outer layer of the blastocyst, called the trophoblast, grows out and mingles with the endometrium, eventually helping to form the placenta. This disk–shaped organ, containing both embryonic and maternal blood vessels, grows to about the size of a dinner plate and can weigh close to 1 kg. Diffusion of material between maternal and embryonic circulations provides nutrients, exchanges respiratory gases, and disposes of metabolic wastes for the embryo. Blood from the embryo travels to the placenta through arteries of the umbilical cord and returns via the umbilical vein.
From the fourth week of development until birth, the placenta, a combination of maternal and embryonic tissues, transports nutrients, respiratory gases, and wastes between the embryo or foetus and the mother. Maternal blood enters the placenta in arteries, flows through blood pools in the endometrium, and leaves via veins. Embryonic or foetal blood, which remains in vessels, enters the placenta through arteries and passes through capillaries in fingerlike chorionic villi, where oxygen and nutrients are acquired. As indicated in the drawing, the foetal (or embryonic) capillaries and villi project into the maternal portion of the placenta. Foetal blood leaves the placenta through veins leading back to the foetus. Materials are exchanged by diffusion, active transport, and selective absorption between the fetal capillary bed and the maternal blood pools.
The heart begins beating by the fourth week and can be detected with a stethoscope by the end of the first trimester. By the end of the eighth week, all the major structures of the adult are present in rudimentary form. (It is during organogenesis that the embryo is most sensitive to such threats as radiation and drugs that can cause birth defects.) At 8 weeks, the embryo is called a foetus. Although well differentiated, the foetus is only 5 cm long by the end of the first trimester.
Meanwhile, the mother is also undergoing rapid changes. High levels of progesterone initiate changes in her reproductive system. These include increased mucus in the cervix that forms a protective plug, growth of the maternal part of the placenta, enlargement of the uterus, and (by negative feedback on the hypothalamus and pituitary) cessation of ovulation and menstrual cycling. The breasts also enlarge rapidly and are often quite tender. (Hmmm)
Second Trimester
During the second trimester, the foetus grows to about 30 cm and is very active. The mother may feel movements during the early part of the second trimester, and foetal activity may be visible through the abdominal wall by the middle of this time period. Hormone levels stabilise as HCG declines, the corpus luteum deteriorates, and the placenta completely takes over the production of progesterone, which maintains the pregnancy. During the second trimester, the uterus grows enough for the pregnancy to become obvious.
Third Trimester
The final trimester is one of growth of the foetus to about 3–4 kg in weight and 50 cm in length. Foetal activity may decrease as the foetus fills the available space within the embryonic membranes. As the foetus grows and the uterus expands around it, the mother′s abdominal organs become compressed and displaced, leading to frequent urination, digestive blockages, and strain in the back muscles. A complex interplay of local regulators (prostaglandins) and hormones (chiefly oestrogen and oxytocin) induces and regulates labour, the process by which childbirth occurs. The mechanism that triggers labor is not fully understood, but shows one model. Oestrogen, which reaches its highest level in the mother′s blood during the last weeks of pregnancy, induces the formation of oxytocin receptors on the uterus. Oxytocin, produced by the foetus and the mother′s posterior pituitary, stimulates powerful contractions by the smooth muscles of the uterus. Oxytocin also stimulates the placenta to secrete prostaglandins, which enhance the contractions. In turn, the physical and emotional stresses associated with the contractions stimulate the release of more oxytocin and prostaglandins, a positive feedback system that underlies the process of labour.
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