The placenta or afterbirth is an amazing organ that forms and grows from a layer of fertilised ovum. These specialised cells fully implant into the wall of the uterus about 12 days after the egg is fertilised, but the placenta is not completely formed and fully functioning until about the 12th week of pregnancy.
The placenta implants flat against the wall of the uterus. By the end of pregnancy it is about the size of a dinner plate and approximately 2cm thick (one inch) and looks very much like a body organ - some people liken it to the human liver.
The placenta acts like a sieve, moving oxygen and nutrients from the mother's body to the baby and taking carbon dioxide and waste materials from the baby into the mother's body for elimination. This intricate process is called diffusion. The blood vessels of the mother and baby are incredibly close together at the site where the placenta is attached to the uterus. However remarkably, the blood flows are always completely separate whilst facilitating this vital exchange.
The placenta allows most substances in the mother's bloodstream to filter across into the baby's bloodstream. Alcohol, nicotine, caffeine, drugs and viruses such as rubella and listeria can enter the baby's blood and affect their growth and development. You can read more in Lifestyle changes for preconception, pregnancy and breastfeeding.
Image 1-10 shows the baby's side of the placenta. It is slippery and smooth and covered in veins.
Image 1-11 shows the side where the placenta attaches to the mother's uterus. It is rough and resembles liver.
Updated November 2007
Stables D. and Rankin J. Physiology in Childbearing with Anatomy and Related Biosciences. 2004, Bailliere Tindall, Edinburgh.