Instant Access to A Level Biology Revision

Sign up now to get access to the entire library of A Level Biology resources for all exam boards

Pregnancy & Menstrual Cycle

The mitotic division initiates as the zygote moves through the oviduct and reaches the uterus. Here it forms two, four, eight, and sixteen daughter cells are known as blastomeres. The embryo with eight to sixteen blastomeres is called morula. The morula proceeds with division and converts into blastocyst as it moves forward in the uterus. The blastomeres in the blastocyst are positioned in an outer layer called trophoblast. An inner group of cells connected to trophoblast is called inner cell mass. The trophoblast layer gets connected to the endometrium, and the inner cell mass forms the embryo.

After attachment, there is a rapid division of uterine cells, and it covers the blastocyst resulting in the embedding of the blastocyst in the endometrium. This is called implantation, and it is the start of pregnancy.

Pregnancy and Embryonic development

Villi and its functions

Finger-like projects appear on the trophoblast after implantation. These projections are called chorionic villi that are surrounded by the tissues of the uterus and blood of the mother.

Placenta and its functions

The villi and uterine tissues connect with each other and form a functional and structural unit between growing embryo (foetus) and maternal body called placenta.

  • The supply of oxygen and nutrients is facilitated by the placenta and also the elimination of carbon dioxide and excretory waste substances produced by the placenta.
  • The placenta is attached to the embryo through an umbilical cord which aids in the transportation of materials to and from the embryo.
  • Placenta also serves as an endocrine tissue and produces several hormones like human chorionic gonadotropin  (hCG), human placental lactogen (hPL), progestogens, estrogens, etc. In the following phase of pregnancy, a hormone called relaxin is also secreted by the ovary.
  • Some toxins, e.g., nicotine, and pathogens, e.g., rubella virus, can pass across the placenta and affect the fetus. Rubella can cause birth defects like cataract, deafness, heart defects, intellectual disabilities, liver and spleen damage, low birth rate, skin rashes, etc. Women who are preparing to get pregnant should get vaccinated before they get pregnant.

Hormones that are produced in women only during pregnancy

  • hCG (human chorionic gonadotropin)
  • hPL (human placental lactogen)
  • Relaxin

Other hormones that are active (rise in levels)  during pregnancy






These hormones are important for the development of the foetus, metabolic changes in mother and regulation of pregnancy.

The inner cell mass (embryo) forms two types of layers

  • Ectoderm (outer layer)
  • Endoderm (inner layer)

Major features of embryonic development

  • Human pregnancy lasts for a maximum of nine months.
  • The heart is formed only after the one month of pregnancy which is seen as the first sign of growing foetus.
  • The limbs and digits are developed in the second month of the pregnancy.
  • Most of the major organs like genital organs and limbs are formed by the end of trimester (12 weeks).
  • During the fifth month, the first movements of the foetus and some hair on the head can be observed.
  • The body is covered with hair, eyelids and eyelashes are formed by the end of the second trimester (24 weeks)
  • The foetus is fully developed by the end of nine months.

Parturition and Lactation

The average period of pregnancy is nine months which is known as the gestation period. Intense contraction of the uterus at the gestation period causes expulsion of the foetus. This process of delivery is called parturition. It is initiated by a complex neuroendocrine pathway.

The signals for the parturition come from the fully developed foetus and the placenta which stimulates mild uterine contractions called foetal ejection reflex.

This further triggers the release of a hormone called oxytocin which results in more vigorous contractions and finally expulsion of the baby through the birth canal.

The placenta comes out of the uterus soon after the delivery.

The mammary glands undergo changes during pregnancy and start producing milk until the end of the pregnancy. The process is called lactation. It is used to feed the newborn. The initial milk of the mother is called colostrum which contains many antibodies that are important for the newborn to develop resistance against diseases.


Breastfeeding is the most important part of the proper growth of the infant and is highly recommended by doctors.

Advantages of breastfeeding

  • It is a natural and great source of nutrition for your child.
  • It is the healthiest food for the babies because it contains antibodies that protect the child from diseases and malnutrition.
  • Breastfeeding is even good for women as it allows the body to recover faster from pregnancy condition and decreases chances of developing arthritis, diabetes, hypertension and heart-related problems.
  • Mother’s milk is easy to digest for infant
  • It is not only convenient and economical source of food for the infant but it also provides comfort to the child.

Disadvantages of breastfeeding

  • Less freedom to mother
  • It can be painful
  • The partner can’t feed and share responsibility
  • It can be difficult in the beginning.

Bottle-feeding formula

These are commercially prepared formulas which are a nutritious substitute to breast milk and contains some vitamins and minerals that are essential for the growth of babies.


  • Convenient to use
  • Another partner can also participate in feeding the child and creates bonding.
  • Mothers can easily feed without the need for private space.
  • Eating habit of mother won’t affect the child
  • These are often fed in less frequency because these are less digestible.


  • Lack of antibodies
  • Do not match the nutritional value and complexity of breast milk.
  • Need to be planned and organised and is not always available like breast milk.
  • Necessary supplies like nipples and bottles are required.
  • It can be quite expensive.
  • In some cases, it also causes gas and constipation problems

Antenatal care

It is the care of mental and physical health during pregnancy. Women should take proper dietary supplements and avoid food that can be toxic to the child.

Supplements like folic acid, vitamin D and iron are essential for the mother. Multivitamins prescriptions should be used in consultation with the doctors, and scheduled visits to the doctor should be complete without delay.

Effect of alcohol consumption

High level of alcohol during pregnancy causes fetal alcohol syndrome. Its consumption also leads to growth restriction, facial anomalies, learning disabilities, and behavioural problems.

Effect of smoking

Smoking in pregnancy causes adverse effects and can result in growth restriction, low birth weight, miscarriage and stillbirth, premature delivery and placental issues.

What happens when an egg is not fertilized?

 Menstrual Cycle

The reproductive cycle in the female is simply called the menstrual cycle. It starts with puberty, and first menstruation is termed as menarche. It repeatedly occurs at an average interval of around 28 days. The period start of one menstruation until the next one is called the menstrual cycle.

One egg is released during the middle of each menstrual cycle. This is called ovulation.

Phases in the Menstrual Cycle

Menstrual Phase – The cycle starts with the menstrual phase

  • Menstrual flow occurs
  • Last for 3-5 days
  • The breakdown of the endometrium lining of the uterus and blood vessels results in menstrual flow. It occurs only if the egg released from ovary is not fertilised.
  • Absence of menstruation might indicate the pregnancy. However, it is not always the case. It can happen due to stress, poor health and some other conditions also.

Follicular Phase

  • The primary follicles in the ovary grow to form a fully mature Graafian follicle
  • Regeneration of endometrium of the uterus
  • Changes in the levels of pituitary and ovarian hormones.

Involvement of hormones in the menstrual cycle

  • Gonadotropins (Luteinising hormone and follicle stimulating hormones) – increases during the follicular phase and initiate growth of follicles and secretion of estrogens by these follicles.
  • Both LH and FSH achieve a peal in the middle of the cycle (14th day).

Ovulation Phase

  • Rapid secretion of LH reaches its maximum level in the mid-cycle. This is called the LH surge. It initiates the rupture of Graafian follicle and results in the release of the egg and thus ovulation.

Luteal Phase

  • Remaining Graafian follicles change into the corpus luteum.
  • The corpus luteum releases large amounts of progesterone which is critical for maintenance of the endometrium.
  • In the absence of fertilisation, the corpus luteum disintegrates. This leads to menstruation.

Menstrual cycle stops around fifty years of age and this event is termed as menopause.