What is Birth?

Legal Definition
Birth, also known as parturition, is the act or process of bearing or bringing forth offspring. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed and breathe. In some species the offspring is precocial and can move around almost immediately after birth but in others it is altricial and completely dependent on parenting. In marsupials, the fetus is born at a very immature stage after a short gestational period and develops further in its mother's pouch.

It is not only mammals that give birth. Some reptiles, amphibians, fish and invertebrates carry their developing young inside them. Some of these are ovoviviparous, with the eggs being hatched inside the mother's body, and others are viviparous, with the embryo developing inside her body, as in mammals.
-- Wikipedia
Legal Definition
The act of being wholly brought into the world. The whole body must be detached from that of the mother, in order to make the birth complete. 5 C. & P. 329; S. C. 24 E. C. L. R. 344 6 C. & P. 349; S. C. 25 E. C. L. R. 433.

2. But if a child be killed with design and maliciously after it has wholly come forth from the body of the mother, although still connected with her by means of the umbilical cord, it seems that such killing will be murder. 9 C. & P. 25 S . C. 38 E. C. L. R. 21; 7 C. & P. 814. Vide articles Breath; Dead Born; Gestation; Life; and 1 Beck' s Med. Jur. 478, et seq.; 1 Chit. Med. Jur. 438; 7 C. & P. 814; 1 Carr. & Marsh. 650; S. C. 41 E. C. L. R. 352; 9 C. & P. 25.

3. It seems that unless the cbild be born alive, it is not properly a birth, but a carriage. 1 Chit. Pr. 35, note z. But see Russ. & Ry. C. C. 336.
-- Bouviers Law Dictionary
Legal Definition
The act of being bom or wholly brought into separate existence. Wallace v. State, 10 Tex. App. 270.
-- Black's Law Dictionary
Legal Definition
The fact of being wholly born, alive.
-- Ballentine's Law Dictionary