To tare the balance is when, for example, you place a beaker on the balance and then click on the tear button which puts the balance back to zero. This putting the balance back to zero is called "tareing the balance".
No, it is an example of positive nitrogen balance. This is because a pregnant women is in need of protein to allow growth of the child, and in order to build protein you need Nitrogen. Therefor pregnancy is considered positive nitrogen balance.
The plural form for the name McCarthy is the McCarthys. Names of people that end in letter -y do not drop the -y to add -ies; for example the two Marys, or the Kennedys (not the Maries and the Kennedies).