11.1 Essential ideas

11.1.4b Sexual reproduction: pregnancy and birth

Fertilisation takes place in the oviduct. After fertilisation, the zygote continues to travel toward the uterus, aided by cilia in the oviduct wall. Meanwhile, it undergoes mitosis, developing into a hollow ball of cells, called a blastocyst.

Endometrial implantation of the blastocyst is essential for pregnancy

Figure 11.1.4b/a – Implantation of a blastocyst, early (left) and late (right)Figure 11.1.4b/a – Implantation of a blastocyst, early (left) and late (right)

  • The blastocyst consists of an inner mass of cells that eventually develops into the embryo, and an outer layer of cells called the trophoblast which eventually develops into the placenta. The cavity of the blastocyst contains a nourishing fluid.
  • By the time the blastocyst has reached the uterus, the zona pellucida has broken down – about five to seven days after fertilisation.
  • The blastocyst settles on the uterine lining (endometrium). Once settled, the cells of the trophoblast are signalled to differentiate in a way that allows further embedding of the embryo.
  • The implanted embryo exchanges oxygen, nutrients and waste products with maternal blood.

HCG stimulates the ovary to secrete progesterone

  • HCG, or human chorionic gonadotropin, is secreted by the developing embryo and can be detected in the maternal bloodstream at the very early stages of pregnancy.
  • HCG stimulates the corpus luteum to continue secreting progesterone – the hormone that stimulates the continued development of the uterine lining.

The placenta facilitates exchange between mother and fetus

  • The placenta is a disc-shaped organ composed of fetal tissue, called chorion, and maternal tissue, called decidua. The chorion develops by the differentiation of cells of the trophoblast. The decidua develops from the endometrial lining.

Figure 11.1.4b/b – Structure of the placenta and amniotic sacFigure 11.1.4b/b – Structure of the placenta and amniotic sac

  • The umbilical cord extends out of one end of the chorion. The outer membrane of the chorion surrounds the amniotic sac, which is filled with a fluid that supports and protects the fetus.
  • Nutrients and wastes are exchanged in the placenta at placental villi. These are projections of fetal blood capillaries, separated from maternal blood by a selectively permeable membrane (placental barrier).
  • The table below summarises the types of transport that occur at the placental barrier.


Direction of movement

Type of transport


In and out


Carbon dioxide











Facilitated diffusion



Endocytosis (active transport)

Estrogen and progesterone are secreted by the placenta once it forms

  • By week 9, fetal secretion of HCG falls drastically. The placenta takes over the role of estrogen and progesterone production from the corpus luteum.

Birth is mediated by positive feedback

  • The hormone responsible for the muscular contractions of parturition is called oxytocin.
  • Oxytocin secretion by the pituitary gland is inhibited by progesterone from the placenta during pregnancy.
  • At the end of a full-term pregnancy, the fetus signals the placenta to stop producing progesterone by releasing prostaglandin hormones.
  • As a result, the pituitary gland secretes oxytocin and the muscular portion of the uterine wall (myometrium) begins to contract.

Figure 11.1.4b/c – Positive feedback in parturitionFigure 11.1.4b/c – Positive feedback in parturition

  • The contractions are perceived by stretch receptors in the uterine wall, which stimulate further secretion of oxytocin by the pituitary gland. This is an example of positive feedback.
  • The contractions become more frequent and powerful over the course of hours. Eventually, the baby is pushed through the cervix and birth canal and is expelled.
  • Oxytocin secretion stops after the birth of the baby – there is little stretching of the myometrium. The placenta detaches from the uterine wall and is expelled about 30 minutes later.

Figure 11.1.4b/d – First breathsFigure 11.1.4b/d – First breaths
With his umbilical cord clamped, this baby takes his first independent breaths.

Key questions

  • Describe the process of implantation.
  • Outline how the structure of the placenta facilitates exchange between mother and fetus.
  • Explain how positive feedback is involved in parturition.

Concept help

  • A blastocyst consists of undifferentiated cells, or stem cells. See 1.2.1.
  • The zona pellucida prevents the blastocysts from implanting in the oviduct.
  • The placenta grows with the needs of the developing fetus. Villi increase the surface area for the diffusion of nutrients and wastes.

Did you know?

  • If the blastocyst implants in the fallopian tubes (oviduct) or cervix, the result is an ectopic pregnancy. Ectopic pregnancies should be terminated, as they can be fatal for the mother if left untreated.

Course link

  • Implantation is only successful when the endometrium is sufficiently mature. Compare the timeline of fertilisation and implantation to the timing of the menstrual cycle. See 6.1.6.