when a solitary sperm (malegametes) penetrates the zona pellucida which surrounds the mature oocyte(female gametes). DNA is released into the focal point of the egg creating 23sets of chromosomes, shaping a zygote.
During the sperms journey throughthe fallopian tube the egg discharges chemotaxis which draws in the sperm. Thesingle sperm goes through an acrosome reaction, achieving fertilisation by puncturingthe eggs cells membrane. At the point of entering a cortical reaction istriggered, hardening the zona pellucida to stop any more sperm getting inside. Day 4 post fertilisation thesingular cell will have divided into a cluster of 16 cells, now becoming themorula. The morula will continue to divide until a formation known as ablastocyst consisting of 32+ cells is created. This subsequently forms theembryo and by day 7 the shedding of the zona pellucida results in implantationinto the growing uterine wall. On completion of implantation thewoman’s body will now go through the stages of pregnancy. Pregnancy normallylasts 40 weeks starting from the woman’s last menstrual period, this is splitinto three phases called trimesters.
First trimester lasts from weeks 1-13,second is from week 14-26 and the third covers weeks 27-40.During the first trimester thecorpus luteum in the ovary produces most of the hormones, oestrogen andprogesterone which contributes to the support of the embryo and development ofthe placenta. However, the corpus luteum degenerates early in the secondtrimester making the placenta the primary hormone producer. Oestrogen is thehormone responsible for most of physiological changes that occur duringpregnancy. The normal menstrual cycle no longer takes place, oestrogensupresses the release of the hormones follicle stimulating hormone (Fsh) and luteinisinghormone(LH) stopping ovulation. High levels of oestrogen also help growth ofthe foetus organ systems, such as the foetal lungs and liver. Oestrogen andhuman chorionic somatotropin (hCS) work together to stimulate maternal tissuegrowth, enables development of the foetus and the maturation of the breastspreparing for lactation and breast feeding. The change in oestrogen levels atthe start of pregnancy is often a contributing factor of symptom’s such as,exhaustion, nausea, vomiting and headaches.
A lot more energy is requiredduring gestation, the growth of the anterior piturity gland causes an increasein production of thyrotropin. Thyrotropin effects the thyroid gland andincreases metabolic rate and appetite, helping the body cope with theadditional calories needed for foetal growth. Human chorionic somatotropin issecreted by the placenta modifies the metabolic state of the body duringpregnancy to facilitate the energy supply of the foetus. Anti-insulin actionsof hcs may also account for the gestational diabetes that develops in 10% ofpregnancies. The average weight gain during pregnancy is between 25-35 pounds.A breakdown of the total weight is: foetus 7-8pounds, placenta and anomaticfluid 6 pounds, breast 3-4 pounds, uterus 4 pounds, fat 3-10 pounds and bloodvolume 4 pounds. The hormone relaxin is responsible for joints and ligamentsbecoming more flexible, to enable easy passage of the foetus through the birthcanal. The increased flexibility can lead to waddling gait and other jointpain.
More blood and plasma are neededin the body to ensure efficient levels of nutrients and oxygen is supplied tothe foetus and mum. Blood volume increases by 40-50%. During pregnancy the riskof venous thromboembolism (VTE) is higher due to the rise of coagulationproduced by the liver. Extra blood puts more pressure on the heart, makingheart rate rise by 10-20 more beats a minute causing increased cardiac output.Normally, the heart beating faster, and increased blood volume would result inhigh blood pressure, but progesterone causes blood vessels to dilate, resultingin lower blood pressure. The respiratory system alsoadapts to pregnancy due to intra-abdominal pressure caused by the growinguterus.
Shortness of breath is experienced in the third trimester, as theuterus grows it pushes the diaphragm 4 centi-meters upwards compressing thelungs. This causes the functional residual capacity to decrease; however, thetotal lung capacity is unaffected. Towards the end of pregnancy, the pressurefrom the uterus eases when the foetus descends into the pelvis. However, thenew location of the uterus applies extra force on the urinary bladder leadingto frequent urination.