Isn’t it amazing that your body can provide everything your growing baby needs to grow big and strong on the inside, until he is ready to meet you on the outside?
The placenta, umbilical cord, amniotic sac, and waters make up a baby’s aquatic environment during the transition, over 40 weeks, from a developing embryo or fetus, to a beautiful bundle of joy!
The placenta provides everything your baby needs to survive and thrive throughout pregnancy, and has a very important role to play.
For more information, you might like to read What Is A Placenta? 13 Amazing Placenta Facts.
The placenta is made up of 3 different layers:
- The amnion
- The chorion
- The decidua
Both the amnion and the chorion together form the fetal membranes which surround and house your baby. The decidua forms the fleshy tissue of the placenta.
Related reading: What Is The Chorion? | All You Need To Know
What is the amnion?
The membranous sac that houses your baby during pregnancy consists of two layers known as the amnion and the chorion.
The human amnion is a thin, nearly transparent membrane which forms one of the layers of the amniotic sac.
The amniotic membrane forms the inner surface of the placenta. It holds your growing baby and the amniotic fluid throughout pregnancy. It creates a closed sac and is a protective sterile bubble around your baby to keep him safe. The chorionic membrane forms the outer layer. It surrounds the amnion and acts as a second protective barrier.
Interestingly, the amnion has no vascularity. That means it has no blood supply of its own. The transfer of nutrients from mom to baby occurs by the process of diffusion.
What is the purpose of the amnion?
The amnion is a thin layer or membrane, which creates a fluid filled cavity there to support your baby as they grow and develop inside the womb. Amniotic fluid is found inside this thin membrane. The fluid acts as a buffer or cushion, and protects your little one from bumps and knocks to the maternal abdomen.
When is the amnion formed?
In human beings, the amnion is formed in very early pregnancy. Of the three fetal cavities in the embryo (the amnion, chorion, and the yolk sac), it’s thought that the amnion is the first to develop. The amnion is formed on the 8th day of embryonic development (8 days following conception).
When the amnion is first formed, it’s in direct contact with the body of the embryo but around the fourth or fifth week of embryonic development, the amnion begins to fill with fluid. As the sac expands, the fluid increases and the baby grows, the amnion (the inner layer) begins to adhere to the surface of the chorion (the outer layer). This occurs around 14-16 weeks of pregnancy.
Why is the amnion important?
Although it’s only a thin layer of tissue, the amniotic membrane needs to be strong and elastic to be able to expand as your baby grows. The membranous sac also needs to be durable enough to withstand pressure created by the fluid inside.
As we know, the waters surrounding your baby in pregnancy are essential for life. Without them, the baby would not be able to survive. Not only does the fluid cushion the baby from the bony structures of the mother’s pelvis, and from external bumps, it has a number of other roles too.
The fluid is important for:
- Umbilical cord protection. It reduces the chances of the cord becoming compressed, which would reduce blood and oxygen flow to the baby
- Temperature regulation. It acts as a form of insulation and helps maintain a constant temperature around the baby
- Muscle and bone development. It allows babies free movement, which means they can float and kick about at their leisure, strengthening their developing bones and muscles
- Lung and digestive system development. When babies practice breathing and swallowing movements during pregnancy, it helps the developing muscles of these systems
- Lubrication. It prevents certain body parts, such as the fingers and toes, from sticking together; it also prevents webbing
- Infection control. The fluid contains immune cells and infection-fighting antibodies from the mother. That means your antibodies can protect and support your baby’s immune system.
What is the difference between the amnion and the chorion?
The main difference between the amnion and the chorion is the way each one functions. The main function of the amniotic membrane is that of protection, whereas one of the main functions of the chorion is to provide support for the embryo and the inner membrane.
The chorion develops forming tiny finger-like projections on its surface; these are known as chorionic villi. The formation of these tiny tissues promotes the exchange of nutrients between mother and baby. This makes sure the mother’s and the baby’s blood supplies are completely separated.
Possible complications – amnion and amniotic fluid
As we know, the waters surrounding your baby are important. But if the fluid levels are too high or too low, this can cause complications for both the mother and the baby.
Fluid levels tend to peak around 34 Weeks Pregnant, and tail off in the later stages of pregnancy, closer to the time of birth.
You can read more about this in our article Amniotic Fluid Level |What You Need To Know.
What happens if your fluid levels are too low?
This is known as oligohydramnios – a condition that arises when there is too little amniotic fluid surrounding the baby. It occurs in 4% of all pregnancies and in 12% of post date pregnancies.
Oligohydramnios is defined when the amniotic fluid index or AFI (the standard measurement used for amniotic fluid levels), is measured at less than 5.
It is most problematic if it occurs in the first 6 months of pregnancy, as this increases the chances of birth defects, pregnancy loss, preterm birth, and neonatal loss.
For more information, please read Low Amniotic Fluid Level – Oligohydramnios.
What are the causes of oligohydramnios?
Low fluid levels can be caused due to premature rupture of membranes, or PROM, where the bag of waters breaks early or before labor begins.
It can also occur due to problems which lower the levels of fluid created, for example:
- Poor fetal growth
- Blood pressure problems
- Placental problems, such as Placental Abruption
- Multiple pregnancies (twins, triplets, etc)
- Post dates
- Birth defects.
What happens if your fluid levels are too high?
Polyhydramnios is a condition that arises when there is too much fluid surrounding the baby. This condition occurs in about 1% of pregnancies.
Polyhydramnios is defined when the amniotic fluid index or AFI, is measured at greater than 24.
Polyhydramnios is associated with a number of fetal complications, such as:
- Poorly controlled diabetes
- Gastro-intestinal disorders
- Central nervous system disorders, such as anencephaly or microcephaly
- Fetal heart rate problems
- Fetal lung complications
- Still birth
- Fetal malposition
- Cord prolapse.
Related reading: What Is Microcephaly? What You Need To Know.
Polyhydramnios can also cause a number of maternal problems too, such as:
- Abdominal pain or difficulty in breathing, due to the increased pressure on the lungs
- Preterm birth
- Premature rupture of membranes (PROM)
- Placental abruption
- Post partum hemorrhage (PPH).
To learn more about polyhydramnios you can read our article Too Much Amniotic Fluid | What You Need To Know.
Amniotic band syndrome
Although rare, sometimes the amnion can be damaged during pregnancy. Part of the amniotic membranes can become damaged, causing strings or amniotic bands – fibrous tissues that can become entangled around the baby.
These amniotic bands can tangle around any body part, but usually affect the baby’s limbs, restricting blood flow to the area. When a band causes damage to any part of the baby’s body, this is known as amniotic band syndrome or ABS.
The severity of the case will depend on the stage of pregnancy when the entanglement occurs, the part of the body that’s involved, and the extent to which the blood flow is restricted or cut off.
In some cases, the blood flow to the affected body part can be severely constricted or cut off completely. This can impede normal movement, growth, and development. In some cases, the restricted blood supply can cause a number of birth defects, or malformations, and in extreme cases can lead to loss of limbs.
Amniotic band syndrome diagnosis
It is thought that amniotic bands are reported in up to 1 in 1,200 live births, but a much smaller proportion actually develops into amniotic band syndrome.
ABS is usually diagnosed after birth during a physical examination of the baby. It can sometimes be detected by ultrasound prenatally, although it is not always obvious enough to see on a scan.
If ABS is detected during pregnancy, a combination of ultrasound, MRI and dopplers will be used to assess blood flow to the area.
Treatment for amniotic band syndrome will depend on the severity and the affected body part.
Options include a special type of fetal surgery, which is performed using fetoscopy. This a special kind of keyhole surgery, where a small incision is made in the uterus and through the amniotic cavity to gain access to the baby. The purpose of the surgery is to relieve the pressure the band is putting on the developing limbs, and to enable normal blood supply to the area.
If this is not possible, or if the amniotic band is not diagnosed until after birth, options for reconstructive and plastic surgery will be discussed. In some cases physical therapy and prosthetics might be required to improve function in the affected limbs.
The amnion and stem cell research
The fetal membranes, including the amnion, are typically discarded as clinical waste at the time of birth. They might, however, be of clinical significance in certain fields of medicine.
There is ongoing research to suggest that the human amnion has the potential to be used in regenerative medicine and cell therapy, to treat damaged and diseased tissues, particularly in the fields of dermatology, orthopaedics, and ophthalmology.
Further research is needed to discover the full potential of these special membranes.