Fetal Development

November 12, 2012

Fetal Development

Fetal development is the term used to describe the process beginning with the fertilization of an egg all the way through to the birth of the baby. There are many stages required for proper fetal development.

Pre-embryonic stage

This stage is the part of the fetal development that includes fertilization and implantation of the egg. Fertilization refers to the action of a man ejaculating sperm into a woman’s vagina during intercourse and when one sperm reaches and enters the ovum. Implantation occurs if the egg travels safely down the fallopian tubes to the uterus. This normally takes from 8-10 days during which time the cell begins to divide becoming a structure known as a blastocyst and attaches itself to the inner uterus. 50 percent of fertilized eggs don’t make it to this stage.

Embryonic stage

This stage is from the third to the eighth week of pregnancy and during this stage of fetal development the germ cells begin to form layers of the body organs. During this period women should be careful to avoid teratogens as they can cause birth defects affecting the major organs such as the heart and kidneys.


Fetal development can be split up into stages according to milestones, which are as followed:

  • 4 weeks

By four weeks the fetus is 0.75 cm in length and weighs 400mg. The spinal cord has formed and arms and legs form buds. The heart is beating and the eyes, nose and ears are just beginning to develop.

  • 8 weeks

At this stage of fetal development, arms and legs have grown, external genitalia is visible although gender is not yet clear and the fetus is 1 inch in length, weighing 20g.

  • 12 weeks

The heart can now be heard with use of an electronic Doppler and there is some very subtle movement. The fetus is now 7-9 cm long and weighs 45g.

  • 16 weeks

The fetus is now anywhere between 10 and 17cm long and weighs between 55 and 120g. At this stage of development a sucking action from the mouth is evident.

  • 20 weeks

By this stage the movement is strong enough to be felt by the mother and the heart beat can now be heard with a stethoscope. The fetus is now up to 25 cm long and 550g in weight.

  • 24 weeks

Eyelashes and eyebrows have now developed and the eye lids will start to open. The pupils become reactive to light and if born now it is possible the baby would survive with neonatal intensive care.

  • 28 weeks

At this stage of fetal development the fetus is around 38cm and weights 1200g and much of the development is happening in the brain.

  • 32 weeks

The fetus now hears and responds to sounds and it may begin to assume the position for delivery.

  • 36 weeks

The central nervous system of the fetus is now developed and able to control body functions.

  • 40 weeks

At this final stage of fetal development, the fetus has now formed fingernails and is kicking and moving rapidly as it is running out of space. At this point the mother is probably is significant discomfort.

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