The kidney is not one large filter. Each kidney is made up of about a million filtering units called nephrons. Each nephron filters a small amount of blood. The nephron includes a filter, called the glomerulus, and a tubule. The nephrons work through a two-step process.
Generally speaking, first babies tend to engage from about 34 weeks to 36 weeks. However, some babies only engage once labour starts. Try not to worry, though. Even if your baby hasn't engaged by the time labour starts, in most cases, it's not a problem.
Since pregancy is actually 9.5 months long because you add the two weeks with it before you actually got pregnant, you take 9.5 and divide it by the number you got from dividing 40 by the number of weeks you are. So...at 16 weeks pregnant, it will be 40/16 = 2.5. 9.5/2.5 = 3.8 calendar months pregnant.