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.
The fact is that there are only a limited number of days each cycle when a woman can get pregnant - the five days before she ovulates and the 24 hours after she ovulates. These six days are the only fertile days of a woman's cycle and they take into account the lifespan of sperm (5 days) and the lifespan of the ovum (24 hours).