What is Chorion?

As we know, the placenta is a vital organ that grows in pregnancy – not just in human pregnancy but in the pregnancy of all mammals. It is an essential organ for the survival and development of the baby.

The human placenta is the only temporary organ, which means once it has performed its function it will be expelled by the body. The placenta attaches to the uterine wall during the first trimester and grows alongside your baby.

Usually, the mother’s and the baby’s blood do not mix. Instead, the placenta acts as a barrier, as well as a means of communication, between mother and the unborn baby.

The placenta is the site of exchange for substances such as oxygen, carbon dioxide, water, nutrients, hormones, and waste products.

Deoxygenated blood flows from the fetus via umbilical arteries to the placenta for exchange, where oxygenated blood is collected and transported back to the fetus via the umbilical vein.

You’d be forgiven for assuming that, as the placenta grows within the mother’s uterus, it must be a maternal organ; in fact, the placenta is a fetal organ.

Even though it doesn’t grow within the fetal body, it’s an organ designed specifically for your baby. This is why a new placenta develops with each pregnancy that you have.

What is the difference between the chorion and the amnion?

The bag of water that is present around your baby as he grows is made up of two layers of membranes. These transparent fetal membranes are known as the chorion and the amnion; together, they form the amniotic sac.

The chorion and the amnion play two different roles.

What is the chorion?

The human chorion forms the outer layer of the fetal membranes. It develops from an outer fold of the yolk sac and is known as the fetal part of the placenta. As the chorion is the outer layer, it surrounds the amnion and the growing fetus. The chorion forms a double protective barrier and adds an additional level of protection for your baby.

Chorion development occurs at a very early gestation – around two weeks post conception – so it happens probably before you even realize you’re pregnant.

What is the amnion?

The amnion forms the inner layer, and sits on the inside of the chorion. It holds amniotic fluid and provides the aquatic environment for your baby. This thin but durable membrane is flexible and has the ability to grow and stretch to accommodate the growing baby contained within.

The amniotic sac grows faster than the chorionic sac. The two membranes, therefore, come into direct contact with each other and subsequently fuse together.

To find out more, read our article about Amniotic Fluid.

What is the purpose of amniotic fluid?

The amnion forms a fluid filled sac around the developing embryo and contains amniotic fluid in which your baby floats freely. The amniotic fluid is required for survival and normal fetal development.

It serves three main purposes:

Protection. The fluid around baby acts as a shock absorber and therefore protects your growing baby from bumps and knocks which could potentially cause injury. It also acts as protection for the umbilical cord, to prevent the cord from becoming compressed and restricting blood and oxygen flow to the baby. The umbilical cord is also essential for the transport of waste products back into the maternal system
Temperature control. The amniotic fluid maintains a constant temperature and helps keep your baby warm. This is also known as thermoregulation
Infection control. Amniotic fluid contains maternal antibodies, which help protect the baby from infection.

The level of amniotic fluid is important for normal fetal development. Too Much Amniotic Fluid can be just as problematic as too little.

Find out everything you need to know about Amniotic Fluid Levels.

Amniotic fluid and stem cell research

The composition of the amniotic fluid surrounding your baby will vary, depending on gestational age. During the latter part of your second trimester, your baby’s urine makes up the majority of the fluid.

However, the fluid also contains:

Water from the mother’s body
Hormones
Electrolytes
Enzymes
Nutrients
Skin cells
Lanugo – soft downy body hair which develops on the fetus for warmth and protection
Vernix – the waxy deposit covering the skin of the newborn
Maternal antibodies
Stem cells.

Amniotic fluid is usually discarded at the birth as medical waste; now, however, researchers believe that amniotic fluid is a rich and ‘untapped’ source of stem cells with therapeutic potential.

Stem cells could have the potential to be grown and to become new tissue that could be used in transplants and treatments in regenerative medicine.

What is the purpose or function of the chorion?

The chorion promotes the exchange of gases – such as carbon dioxide and oxygen – and nutrients between mother and baby. It does this via a very special mechanism.

As the chorion develops, finger-like projections begin to grow out. These are known as primary chorionic villi and, within these, secondary villi develop. These begin to grow in the second to third week of gestation.

As these finger-like projections, or folds, develop, there’s an increase in the surface area in contact with the mother’s blood.

Tiny fetal blood vessels, or capillaries, begin to grow from the chorionic villi and connect to the embryo’s heart. Maternal blood flowing through these capillaries provides oxygen and nutrients to the growing baby. The villi continue to grow and become the fetal placenta.

What is the purpose of chorionic villi sampling?

Chorionic villi sampling, also known as chorionic villus sampling (CVS), is a prenatal test used to detect chromosomal or genetic abnormalities, such as Down’s syndrome, Edward’s syndrome, or Patau’s syndrome.

It’s important to note that CVS is a diagnostic test, rather than a screening tool.

Diagnostic tests are used to detect certain genetic conditions, whereas screening tests, such as the Nuchal Translucency Scan, are used to evaluate an individual’s risk or chance of developing a specific genetic condition.

When is CVS offered?

CVS isn’t offered routinely to everyone; it is done only if there’s a high chance a baby might have a genetic or chromosomal abnormality.

You could be offered CVS if:

An antenatal screening test has suggested your baby might have a genetic or chromosomal condition
You’ve had a previous pregnancy affected by a genetic condition
You are over 35 years old
You or your partner are known to be a carrier for certain genetic conditions, such as cystic fibrosis
There is an increased risk of sex-linked genetic conditions.

CVS can be undertaken by inserting a thin catheter through the cervix, or by carefully inserting a needle into the abdomen. Most practitioners usually prefer the abdominal route.

The CVS is usually performed at about 10-13 weeks gestational age.

What’s the difference between amniocentesis and chorionic villi sampling?

Amniocentesis, or an ‘amnio’, is another type of diagnostic test you might have already heard about.

An amnio involves taking a small sample of amniotic fluid from around the baby. The amniotic fluid contains fetal cells, substances, and genetic material from the baby, which can be used to detect various chromosomal or genetic conditions.

An amniocentesis can be used to detect:

Genetic conditions such as Down’s syndrome
Infection
Neural tube defects (NTDs)
Fetal blood typing
Lung development.

An amniocentesis is performed under the guidance of an ultrasound, and therefore does not harm your baby. Your baby will make more fluid to replace what has been taken out during the procedure.

An amniocentesis is performed slightly later in the pregnancy – usually during the second trimester, at about 15-18 weeks gestation.

One of the advantages of chorionic villi sampling is that it can be performed earlier in the pregnancy, at about 10-13 weeks. This means test results can be given earlier and decisions regarding care can be made sooner.

Learning of abnormalities before birth can help guide parents in making better informed decisions about care.

For some, an amniocentesis might be better option – if, for example, you or your partner have a neural tube defect, or if you have had a previous pregnancy affected by a NTD. The CVS provides no information on NTDs, such as spina bifida.

What are the risks of CVS and amniocentesis?

As with any surgical procedure, both CVS and amniocentesis carry a certain number of risks.

Both procedures involve similar risks, including:

Infection. There is a risk of infection, as with any surgical procedure
Bleeding/spotting. You might experience minor bleeding or spotting following the procedure; this could appear red or brown in color
Leaking of amniotic fluid. Slight leaking of fluid can occur following an amniocentesis. If you are not sure whether or not you’re leaking fluid, read our article Is Amniotic Fluid Sticky? 9 Top Questions Answered for more information
Cramping. It’s common to experience some period-like pains afterwards
Rh sensitization. During CVS, it’s possible for some of the baby’s blood to enter the maternal bloodstream. If you have a rhesus negative (Rh–) blood type, you might be offered an injection (Anti-D) to stop your body developing antibodies against the baby’s blood. Read our article on Rhesus Factor And Pregnancy for further information
Preterm Labor or miscarriage.

It is thought the risk of miscarriage following CVS or amniocentesis is 1 in 100 (1%). However, it is difficult to determine who would have gone on to have a spontaneous miscarriage, despite the procedure.

If you have concerns about miscarriage, familiarize yourself with Signs of Miscarriage | 5 Signs To Know.

What are the risks to the chorion?

Although it happens rarely, it’s possible to develop an infection following either CVS or amniocentesis. An infection in the fetal membranes or in the amniotic fluid surrounding your baby is known as chorioamnionitis.

For more information, read our article Chorioamnionitis | What You Need To Know And D.

Although it can occur after either procedure, chorioamnionitis can also occur at any stage in pregnancy, or if your waters have been broken for a prolonged period of time.

It is caused by a bacterial infection, which usually starts in the mother’s urogenital tract (urinary tract, vaginal, anus, or rectum) and then travels upwards to the uterus.

If left untreated, an infection within the membranes or in the amniotic fluid can cause complications for both the mother and the baby.

For the mother, these can include:

Pelvic infections (in the endometrium or uterus)
Preterm labor and birth
Blood clots in the pelvis or lungs
Sepsis.

For the baby, complications might include:

Preterm Birth and associated complications of prematurity
Sepsis
Meningitis
Respiratory problems.

Chorioamnionitis can sometimes be treated with antibiotics; however, birth is often required to prevent further complications, particularly for the newborn.

It is common for newborns to be treated with IV antibiotics if infection is either suspected or confirmed.

Signs of chorioamnionitis include: