Foetal growth and development
Since foetal weight corresponds approximately to the cube of length, foetal weight almost increases with the cube of gestational age.
Initial development of the placenta and membranes occurs much faster than foetal growth. In fact, during 2-3 weeks after implantation, the foetus remains mostly microscopic, but then, as shown in the figure, the length of the foetus increases with age. At 12 weeks, the length is about 10 cm; at 20 weeks is 25cm, and at 40 weeks 53cm. Since foetal weight corresponds approximately to the cube of length, foetal weight almost increases with the cube of gestational age.
Figure. Foetal growth.
Notice in the picture that the weight is still very small during the first 12 weeks and reaches 1 pound (1 pound = 0.45359237 Kg) in just 23 weeks of pregnancy. Then, during the last trimester, the foetus grows rapidly, so that 2 months before birth, the average foetal weight is 3 pounds; 1 month before birth, 4.5 pounds on average; and at birth, averaging 7 pounds, ultimately foetal weight at birth varied from as low as 4.5 pounds to as high as 11 pounds with normal babies through normal gestation.