Placenta and umbilical cord
The placenta connects the developing baby to its mother via the umbilical cord. It supplies your baby with oxygen and food, and removes waste products for disposal via the mother's kidneys.
The placenta develops from the same sperm and egg cells that form your baby. Development is a continuous process that starts at the time of fertilisation and continues throughout pregnancy.
The placenta is usually shaped like a disc and weighs around 500g at term. It connects to your baby via the umbilical cord which usually has two arteries and one vein surrounded by thick protective jelly.
Blood vessels branch out across the placenta, dividing into smaller and smaller vessels ending in villi. The villi are bathed in the mothers blood supply and it is here that your baby's waste products pass from their blood to yours. At the same time oxygen and nutrients pass from your blood to your baby. The blood then travels back to your baby via the umbilical vein. At no point does your baby's blood mix with your blood.
Functions of the placenta
Nutrition: the placenta provides your baby with oxygen and nutrients so that they can grow and develop normally. It is also responsible for removing your baby's waste products and passing it to the mother's blood supply for removal via her kidneys.
Protection: the placenta is not a perfect filter and cannot identify good substances from bad. This is why women are asked to avoid potentially harmful substances such as alcohol and cigarettes while they are pregnant.
Hormones: the placenta produces numerous hormones which are necessary for the pregnancy to continue such as Beta-hCG and progesterone.
In some pregnancies a problem occurs with the way in which the placenta attaches to the uterus, develops, grows or functions. Placental problems are among the most common complications reported in the second half of pregnancy.
This refers to when the placenta peels away from the uterine wall, either partially or fully, before the birth of your baby. Mild cases may cause few problems, however severe cases can be life threatening for both you and your baby. Severe cases are usually characterised by abdominal pain and vaginal bleeding.
Placental abruption usually occurs in the last trimester of pregnancy and affects approximately one in 100 pregnancies. Abruption is a clinical diagnosis and ultrasound may not detect all cases of abruption. Treatment usually involves delivery of your baby and outcome depends on the severity of the abruption and the gestation of the pregnancy.
This is the term used to describe when a placenta is implanted near to, or over, the cervical canal. As the cervix dilates at the start of labour the blood vessels that connect the placenta to cervix/uterus can tear resulting in bleeding. This bleeding can be severe and endanger both mother and baby if the labour is not stopped. Ultrasound is used to diagnose this condition and to measure the distance of the placental edge from your cervix. This information is then used by the obstetrician to advise whether a caesarean birth is necessary.
Normally the placenta is formed within the innermost lining of the uterus. Occasionally a placenta invades the uterine wall and this is called placenta accreta. Placenta increta and percreta refer to a placenta that invades even further, sometimes all the way through the uterine wall and into other organs such as the bladder. This means the placenta will not separate from the uterus in the normal way after the birth of the baby and can cause severe blood loss. The placenta is usually surgically removed to stop the bleeding, and often a hysterectomy (removal of the uterus) is necessary. Placenta accreta affects approximately one in 2500 pregnancies and may be diagnosed by ultrasound in high risk pregnancies (e.g. previous or current placenta praevia).
Where there is a concern about your baby's growth the doctor may request Doppler ultrasound of the umbilical artery. This form of ultrasound measures the flow of blood through the umbilical artery and estimates how much resistance there is within the placenta to the flow of blood.
When the placenta is working well the blood flows easily between your baby and the placenta. However sometimes there are problems within the placenta which causes resistance to the flow of blood—known as uteroplacental insufficiency. This means your baby has to work hard to keep the blood flowing and receive the nutrients it requires. If this pattern continues or there are significant concerns with your baby's growth the doctor may consider delivering your baby early. For further information about fetal growth restriction refer to Mater Mother's Hospitals brochure: Your baby's growth scan.
Umbilical cord problems
Velamentous umbilical cord
This is when the umbilical cord inserts into the edge of the placenta where it does not have the thick protective jelly surrounding it. This affects approximately one per cent of pregnancies and is more common in low lying placentas than normally located placentas. The main concern is the compression of the vessels during labour causing distress in your baby. Diagnosis is usually made after the delivery of the placenta; however it can be diagnosed by ultrasound.
When one or more of the blood vessels from the umbilical cord are located in the placental membranes in front of the cervix. The blood vessels are in danger of rupturing when the membranes break or when the cervix starts to open. They are also at risk of being compressed during delivery (as there is no protection around them) causing decreased oxygen to your baby. If bleeding occurs from a vasa praevia, your baby becomes anaemic. Any bleeding occurring in pregnancy should be investigated. Transvaginal ultrasound accurately diagnoses vasa praevia provided the sonographer is specifically looking to identify the condition. If vasa praevia is diagnosed, caesarean birth is often recommended.
Single umbilical artery
Approximately one in every 100 pregnancies has only one artery and one vein in the umbilical cord as opposed to the normal two arteries and one vein. It is even more common in twin pregnancies, affecting approximately five per cent of pregnancies. The cause is unknown and most babies are born without any problems. However, babies born with a single umbilical artery are at a slightly higher risk of having a birth defect or being smaller than average compared to babies with two umbilical arteries. If other anomalies are found, chromosomal testing may be offered. A growth scan is sometimes performed in the third trimester if the mother is worried about the size of her developing baby.