Normally, the placenta is an overlooked item when a woman is focusing on her pregnancy. It's quite an interesting organ however. It helps filter nutrients and oxygen to the baby, and removes waste products. The placenta is the workhorse that grows with the baby, and provides very important functions to the growth and health of the developing baby.
With most pregnancies, the placenta is normal. The cord inserts towards the center of the placenta. There are 3 vessels that are in the umbilical cord that connect the baby to the placenta (2 arteries, 1 vein), and wharton's jelly helps cushion those blood vessels in the cord. The placenta does it's job, with little to no recognition, and is examined and thrown away after the birth.
However, sometimes the placenta isn't perfect. Developmental changes occur during implantation or during the growth of the placenta and baby. Here are just a few: (caution --- graphic placenta pictures to follow!)
Here we have a twin gestation placenta, or rather, two placentas that have fused together. The interesting part about the top placenta is that it has an accessory lobe, also known as asuccenturiate lobe. You can see the blood vessels connecting the accessory lobe to the main placenta. Sometimes the lobe is connected closer to the main placenta, even contained within the membranes, so that it looks like an extra chunk of placenta on the side. It can potentially be dangerous in the above situation, if the vessels that connect the placenta to the lobe should rupture. Hemorrhage can occur, with fetal distress or even death.
Next up is a battledore cord insertion. This is where the umbilical cord has inserted along the edge of the placenta, instead of a more central location. This can have ramifications of poor blood circulation, since the edge of the placenta is not quite as rich in blood vessels. However, many times this doesn't affect the baby. I've seen several battledore insertions.
This is circumvallate of the placenta. The top placenta shows a partial ring of thickened tissue from the membranes along the edge of the placenta. The bottom picture (above) shows a complete ring of tissue along the edge. Usually doesn't cause a problem, unless there is also a battledore cord insertion along the affected edge. Then you might see a reduction in blood perfusion and circulation due to decreased blood vessels in this area.
Lastly, for today at least, is one of the more interesting placental abnormalities: velamentous insertion. This is where the blood vessels are not contained within the umbilical cord (where they normally are) and instead are found in the membranes leading off of the placenta. This can be a very dangerous situation if the membranes rupture or are artificially ruptured in labor, and the rupture location is very close to or at the vessels in the membranes. The baby can quickly exsanguinate and die. The mother could hemorrhage and also could die. I have also seen this abnormality several times, and have breathed a sigh of relief that the outcomes have been good. Another potential issue with this type of placenta is that the cord insertion is not very strong, and any tugging or pulling on the placenta after the baby is born can lead to a separation of the cord from the placenta. This would then require a manual extraction of the placenta, where the provider needs to reach into the uterus and peel the placenta from the uterine wall. This, of course, increases a risk for infection for the mother, and antibiotics would be started to prevent infection.
Of course, there are other placenta abnormalities, but I thought I would just highlight a few of my "favorites" here today. For more info and reading on placentas and umbilical cords, I refer you to this interesting site: UNSW Embryology