Your Baby's Life Support System
Normally, the development of the fetus, from a tiny group of just a few cells to a fully grown baby, ready for life in the outside world, occurs inside the uterus with the help of the placenta, the umbilical cord and the straw-coloured amniotic fluid. The main functions of these important elements of your baby's support system are explained below.
The placenta is a disc-shaped structure that is attached to the wall of your uterus and is connected to your baby via the umbilical cord. Essentially, it consists of a network of arteries and veins, from your baby, that interfaces with your circulation. At term, it weighs approximately one sixth as much as your baby.
Blood vessels from your baby (the umbilical arteries) enter the placenta, where they divide into fine blood vessels (capillaries) that enter tiny finger-like projections of the placenta (the placental villi). These villi increase the surface area of the placenta and are bathed in your blood, which is transported to the uterus by branches of your uterine arteries (the spiral arteries). This means that your blood and your baby's blood can come very close to each other without actually mixing.
The placenta is usually situated well out of the way of the cervix but occasionally remains in the lower part of the uterus, a condition known as placenta praevia. This may be noted at the 20-week scan, but in nine out often cases, it will have moved out of the way before you are due to give birth.
The placenta has two main functions:
- It enables carbon dioxide and waste products to cross from your baby's blood into your blood.
- At the same time, it enables oxygen and nutrients to cross from your blood into your baby's blood.
In addition, the placenta manufactures a number of hormones:
- Human chorionic gonadotrophin (HCG), which is important for sustaining the pregnancy during the first few weeks (and is responsible for making your pregnancy test positive).
- Human placental lactogen (HPL), which is important for fetal growth.
- Oestriol (a type of oestrogen).
- Progesterone, to maintain the lining of the uterus.
The placenta also protects your baby in the following ways:
- It prevents your baby being rejected by your antibodies.
- It blocks the passage of many (but unfortunately not all) potentially harmful substances, for example drugs and other chemicals.
The umbilical cord
The cord is usually about 50 cm long, 1-2 cm wide and contains two arteries and one vein, which coil around each other in a spiral fashion. Inside the cord is a jelly-like substance called 'Wharton's jelly' which cushions the blood vessels of the umbilical cord. Occasionally there is a knot in the cord. In most cases this does not cause any problems because blood is still able to pass through it.
The amniotic fluid
Throughout pregnancy, your baby is surrounded by a pool of fluid. For most of the pregnancy, this amniotic fluid comes from your baby's urine. The quantity of fluid increases from 150-200 ml at 16 weeks to 1000 ml at 36 weeks.
Amniotic fluid is constantly being recirculated: your baby swallows it as she practises breathing and then passes it out as urine. Any process that interferes with your baby swallowing or passing urine can produce too much amniotic fluid (polyhydramnios) or too little (oligohydramnios). The quantity of amniotic fluid can be measured by ultrasound: a normal amount is one indication that she is thriving.
The amniotic fluid has several functions:
- It allows your baby to move around freely. "It protects her from knocks.
- It keeps her at a constant warm temperature. "It helps her lungs to develop normally.
In a small number of pregnancies, the cord winds around the baby's neck (referred to as a nuchal cord). This rarely causes any complications. However, as the baby descends during labour, the cord may tighten, particularly if it is wrapped more than once around the neck. This may lower the baby's heart rate, resulting in signs of fetal distress. Once the baby's head is out, the midwife can usually gently loop the cord over it. Because it is the baby's lifeline, it is not usually advisable to cut it at this stage.
When your waters break
When your waters break, before or during labour, it is amniotic fluid that is expelled. This is usually clear and straw-coloured, but can be greenish in colour, indicating that your baby has opened her bowels in the womb, producing meconium (a mixture of waste secretions, cells and pigments). This is often a sign of fetal maturity, but may also indicate fetal distress.