Babies need to be given food and water to continue development. Sperm need to fertilize an egg to continue development. Everything is just another stage in development.
A mature human does not need anything to continue development (since development is complete).
Anyway, it all depends on your definition of "human" so whatever. I use "sapient/intelligent" as my definition. A sperm, an egg, an embryo, a fetus, and a baby all will become sapient/intelligent if and only if they are put in certain conditions.
A mature human may not need anything to continue development, but a child or even an adolescent sure do. Hell, in some cases that food still has to be given to them since they cannot obtain it on their own.
This is my own opinion in reference to abortion and a fetus and all of that good stuff.
If I recall correctly something is considered to be alive when it exhibits most of the following qualities.
1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
2. Organization: Being structurally composed of one or more cells, which are the basic units of life.
3. Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
4. Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
5. Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.
6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and by chemotaxis.
7. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.
A developing embryo exhibits all of these qualities besides the 7th, which they will gain as they develop overtime.
Throw that in along with human DNA being a part of what they are, and I am not sure how you can make an argument about a fetus being alive OR human.