22 - Milestones of Prenatal development

1^st week of gestation : fertilization and implantation , beginning of embryonic period.

2^nd week of gestation : endoderm and ectoderm appear ( bilaminar embryo ).

3^rd week of gestation : first missed menstrual period, mesoderm appears ( trilaminar embryo ), somites begin to form.

4^th week of gestation : neural folds fuse, folding of embryo into human like shape, arm and leg buds appear , crown-rump length is 4 to 5 mm .

5^th week of gestation : lens placodes,primitive mouth,digital rays on hands.

6^th week of gestation : primitive nose , philtrum ,primary palate , crown-rump length 21 – 23 mm

7^th week of gestation : eyelids begin

8^th week of gestation : ovaries and testes distinguishable

9^th week of gestation : fetal period begins, crown-rump length is 5 cms, weight is 9 grams .

10^th week of gestation : external genitalia distinguishable

20^th week of gestation : usual lower limit of viability ,weight is 460 grams , length is 19 cms

28^th week of gestation : eyes open , fetus turns head down , weight is 1000 grams

38^th week of gestation : term.