The prenatal stages are separated into three stages called trimesters.

In the first trimester it is the most crucial the development of the major organs. The developing organism is the most venerable to radiation, the effects of drugs and bacterial based infections. From time of conception to 12 weeks.

Second trimester 12 weeks to 24 weeks, this is the time where all the organs are forming.

Third Trimester represents the period of the fetus growing. By this stage the organs are becoming fully functional. 24 weeks to 38 weeks.

	Week 3 Day 15-21 Eyes and ears begin to form. The embryo is 2.5mm(0.1in.) long.

	Week 4 Day-28 Heart starts beating. Tiny bumps appear where arms and legs will grow. There are about 10,000 cells. The embryo is 6.3mm(¼ in.) long.

	Week 5 Day 29-35 Nose and stomach begin to form. The embryo is 13mm(½ in.) long. Day 31 Arms, hands and shoulders begin to take shape. Day 33 Outline of fingers appear.

	Week 6 Stomach, intestines, liver, lungs and brain all developing. The embryo is 19mm (¾in.) long. Tip of the nose appears. Eyelids begin to form. Outline of toes can be seen.

	Week 7-9 Kidneys and liver start functioning. The ears are almost complete. There are definite palm prints and footprints. Nails begin to grow. The fetus is 4cm (1 ½ in.) long.

	Week 11-14 All the main organs are now developed and the baby can frown, swallow and urinate. He or she now weighs 21g(¾ ounce) and is 7cm(2¾ inches) long.

	Week 15-18 Eyelashes and eyebrows begin to grow. The mother can probably feel her baby kicking for the first time.

	Week 19-26 If a baby is born now it is possible, with modern hospital facilities, that he or she might survive at 30cm(12 in.) long and weighing about 0.62kg(1lb 6oz). Pictures of babies in the womb showing them sucking their thumbs. Many babies born now can survive.

	Week 38 The baby continues to put on weight. On average babies are born 266 days after the egg is fertilized. They weigh about 3.3kg(7lb 4oz) and are 51cm(20in) long. They just have half a pint of blood but the heart pumps 340 litres/600 pints through it a day.