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.
A woman with a 35 day cycle will be fertile on days 18-24, with the most fertile days being days 20, 21, and 22. Since sperm can live up to 48 hours inside the body (72 hours if they are super mega swimmers!), a woman could technically have sex a couple of days before ovulation and still get pregnant.