Changes occuring in embryo during pregnancy

Days	Changes
1	Fertilization. Diameter of egg is about 0.15 mm
2	2 cell stage
3	16 cell stage (morula)
4	Entry of blastocyst in the lumen of uterus. Disappearance of zona pellucida. Diameter of blastocyst is about 0.3 mm
7-8	Partial entry of blastocyst inside endometrium; Implantation
12	Complete entry of blastocyst into endometrium. Extra-embryonic mesoderm, amnion and yolk sac
14	Primitive streak formation
18	Formation of 3-5 pairs of somites
19	Neural groove, neural plate, notochordal plate and 6-8 pairs of somites formed.
24	Indication of formation of head and tail region. 21-23 pairs of somites formed. Formation of heart continues at ventral side.
28	Heart starts beating. Neural tube is formed. Blood islands appear. 3 pairs of visceral arch and 30-31 pairs of somites formed.
32	30-39 somites formed
7 weeks	Jaws, fingers and external ears begin to appear. The C-R length (crown-rump length: the length from head to the bottom of hips) is 19-20 mm
8 weeks	Embryo is completely surrounded by amnion. Fingers and toes clearly visible. Almost all organs formed with continuing development. At the end of 8th week the embryo appears like a little human (foetus). C-R length: 28-30 mm
12 weeks (first trimester)	The major organs (limbs, external genital organs etc) are well developed.
5 months	First movement of foetus and appearance of hair on the head. Blood formation starts in bone marrow. Decidua capsularis and parietalis connect together.
24 weeks (second trimester)	Body is covered with fine hair, eyelids separate and eye lashes are formed.
9 months	Placenta attains maximum size. Nails on finger appear. In the next 10 days the foetus is ready to born as a little baby.