Fertilisation, Development, and Gestation

This lesson covers: 

  1. The process of fertilisation and early embryo development
  2. What happens during gestation

Fertilisation and early embryo development

Several processes happen in the body before pregnancy can occur:

  1. Ovulation - This happens on day 14 of the menstrual cycle, when an egg cell is released by an ovary.
  2. Copulation - During sexual intercourse, the male ejaculates millions of sperm into the female's vagina.
  3. Fertilisation - The egg cell and one sperm cell meet and fuse in the fallopian tube, forming a fertilised cell called a zygote.
  4. Cell division - After fertilisation the zygote starts dividing. When it reaches 32 cells, it is called an embryo.
  5. Implantation - The embryo reaches the uterus and implants into the uterine lining.

Gestation

After implantation the embryo continues to grow into a fetus. The development period inside the uterus is called gestation.

Illustration showing the development of an embryo at 1 month and a full-term fetus at 39 weeks.
  • At 1 Month - The embryo measures about 6 mm in length, with the brain, heart, eyes, ears, and limbs beginning to form.
  • At 9 Weeks - Now around 25 mm long, all major body systems have started to form, and the term 'fetus' is used to describe the developing baby.
  • At 3 Months - The fetus has grown to about 54 mm, and has facial features and body proportions.
  • At 5 Months - Measuring around 160 mm, the fetus is active. Fine details like fingernails are also developing.
  • At 7 Months - The fetus is about 370 mm long and has reached a stage of development where it could survive outside the womb under medical care.
  • At 39 Weeks (Full Term) - The baby, approximately 520 mm long, is fully developed and ready for birth.