when a solitary sperm (male
gametes) penetrates the zona pellucida which surrounds the mature oocyte
(female gametes). DNA is released into the focal point of the egg creating 23
sets of chromosomes, shaping a zygote.
During the sperms journey through
the fallopian tube the egg discharges chemotaxis which draws in the sperm. The
single sperm goes through an acrosome reaction, achieving fertilisation by puncturing
the eggs cells membrane. At the point of entering a cortical reaction is
triggered, hardening the zona pellucida to stop any more sperm getting inside.
Day 4 post fertilisation the
singular cell will have divided into a cluster of 16 cells, now becoming the
morula. The morula will continue to divide until a formation known as a
blastocyst consisting of 32+ cells is created. This subsequently forms the
embryo and by day 7 the shedding of the zona pellucida results in implantation
into the growing uterine wall.
On completion of implantation the
woman’s body will now go through the stages of pregnancy. Pregnancy normally
lasts 40 weeks starting from the woman’s last menstrual period, this is split
into 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 the
corpus luteum in the ovary produces most of the hormones, oestrogen and
progesterone which contributes to the support of the embryo and development of
the placenta. However, the corpus luteum degenerates early in the second
trimester making the placenta the primary hormone producer. Oestrogen is the
hormone responsible for most of physiological changes that occur during
pregnancy. The normal menstrual cycle no longer takes place, oestrogen
supresses the release of the hormones follicle stimulating hormone (Fsh) and luteinising
hormone(LH) stopping ovulation. High levels of oestrogen also help growth of
the foetus organ systems, such as the foetal lungs and liver. Oestrogen and
human chorionic somatotropin (hCS) work together to stimulate maternal tissue
growth, enables development of the foetus and the maturation of the breasts
preparing for lactation and breast feeding. The change in oestrogen levels at
the start of pregnancy is often a contributing factor of symptom’s such as,
exhaustion, nausea, vomiting and headaches.
A lot more energy is required
during gestation, the growth of the anterior piturity gland causes an increase
in production of thyrotropin. Thyrotropin effects the thyroid gland and
increases metabolic rate and appetite, helping the body cope with the
additional calories needed for foetal growth. Human chorionic somatotropin is
secreted by the placenta modifies the metabolic state of the body during
pregnancy to facilitate the energy supply of the foetus. Anti-insulin actions
of hcs may also account for the gestational diabetes that develops in 10% of
pregnancies. The average weight gain during pregnancy is between 25-35 pounds.
A breakdown of the total weight is: foetus 7-8pounds, placenta and anomatic
fluid 6 pounds, breast 3-4 pounds, uterus 4 pounds, fat 3-10 pounds and blood
volume 4 pounds. The hormone relaxin is responsible for joints and ligaments
becoming more flexible, to enable easy passage of the foetus through the birth
canal. The increased flexibility can lead to waddling gait and other joint
More blood and plasma are needed
in the body to ensure efficient levels of nutrients and oxygen is supplied to
the foetus and mum. Blood volume increases by 40-50%. During pregnancy the risk
of venous thromboembolism (VTE) is higher due to the rise of coagulation
produced by the liver. Extra blood puts more pressure on the heart, making
heart rate rise by 10-20 more beats a minute causing increased cardiac output.
Normally, the heart beating faster, and increased blood volume would result in
high blood pressure, but progesterone causes blood vessels to dilate, resulting
in lower blood pressure.
The respiratory system also
adapts to pregnancy due to intra-abdominal pressure caused by the growing
uterus. Shortness of breath is experienced in the third trimester, as the
uterus grows it pushes the diaphragm 4 centi-meters upwards compressing the
lungs. This causes the functional residual capacity to decrease; however, the
total lung capacity is unaffected. Towards the end of pregnancy, the pressure
from the uterus eases when the foetus descends into the pelvis. However, the
new location of the uterus applies extra force on the urinary bladder leading
to frequent urination.