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.
In general, there are 5-6 fertile days each month: The day of ovulation and the 5-6 days before ovulation. So there are about 6 fertile days each month, 6 days where you are able to get pregnant, 6 days on which making love you can get pregnant. Assuming you have a 28 day cycle, then you are expected to ovulate about 14 days before the next menstrual period or about 14-15 days into the cycle.