Overview

The placenta is an organ with a highly specialized purpose, and that is to support the normal growth and development of your baby. Before the widespread use of ultrasounds, few expecting mothers knew what position their placenta was in or if it was considered “normal.” When you have your anatomy scan ultrasound, usually between weeks 18-22, the ultrasound technician will examine your placenta. There are many variations in size and placement. Some of these abnormalities are simply variations that have no impact on your baby, and others can signal potential complications your care provider will want to monitor. This article includes information on low-lying placenta, placenta previa, placenta accreta spectrum, bilobed placenta, succenturiate placenta, circumvallate placenta, circummarginate placenta, placenta membranacea, ring-shaped placenta, placenta fenestrata, and abnormal cord insertion.

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This article covers your placenta and some variations you could discover during an ultrasound. Many expecting mothers who have a placenta or umbilical cord that differ from normal have no symptoms, and not all of these diagnoses indicate there will be complications.

The Placenta

After a fertilized egg implants in your uterus, part of those cells develops into the placenta. In mammals, placentas go back to around 150 million to 200 million years ago. This is a flat oval-shaped organ that becomes your baby’s life support system. One side attaches to your uterine wall; the other side has an umbilical cord that connects to your baby.

At birth, a typical placenta is about 8.6 inches (22 cm) in diameter and .78-1 inch (2-2.5) cm thick and weighs about one pound (.45 kg). In most pregnancies, the placenta attaches to the top or the side of the uterus.

The Umbilical Cord

The umbilical cord is your baby’s lifeline. This will be about 20 inches (50-70 centimeters) long and .75 inches (2 cm) in diameter at birth. The cord contains the umbilical vein and two umbilical arteries. The umbilical vein carries nutrient-rich, oxygenated blood from the placenta to your baby. The umbilical arteries carry deoxygenated, nutrient-depleted blood from your baby to the placenta. The umbilical cord typically comes from around the center of the placenta, with the other end attached to your baby.

The Function of the Placenta

The placenta is an organ with a highly specialized purpose, and that is to support the normal growth and development of your baby. Oxygen and nutrients transfer from you to your baby. Carbon dioxide and other waste products transfer from your baby through the placenta and to your blood supply.

Beginning around week 20 of your pregnancy, antibodies pass through the placenta to help protect your baby in utero. The antibodies will help protect your baby during the first few months of life and build their immune system.

Your placenta also plays a significant role in secreting hormones that are vital for your baby. This includes hCG, commonly known as the pregnancy hormone, estrogen, progesterone, oxytocin, and prolactin. These hormones are essential for your baby’s development, and everything going on in your body during your pregnancy and birth and prepare you for breastfeeding. Lastly, your placenta acts as a blood reservoir for your baby and continues to transfer blood even after birth. This is the reason many parents choose to delay cord clamping. Your placenta is an amazing organ; your baby’s life depends on it.  

The Third Stage of Labor

After your baby is born, you begin the third stage of labor, which is when you birth your placenta. As your uterus continues to contract and shrink, your placenta will detach from your uterine wall. The blood vessels close off, and you push the placenta out. Once your placenta is out, your care provider will examine it to make sure everything looks good and that it is intact to ensure all of it has come out. If you have a cesarean birth, your placenta also needs to come out, and your doctor removes this through the incision after your baby is born.

You have several options as to how the third stage of labor is managed and what interventions if any, your care provider utilizes. See this episode for more information on the third stage of labor, including the evidence on interventions and complications. See this episode for information on placenta encapsulation and the research on the risks and benefits of encapsulating or consuming your placenta after birth,

Placenta in Twin or Multiple Pregnancies

If you are pregnant with twins or multiples, your pregnancy will be classified by the type of twins or multiples and how they develop in relation to the amniotic sac and placenta.

Dizygotic twins are when two eggs are released, and both are fertilized and implanted. This results in two separate placentas since the placenta starts to form when the egg implants. These are classified as dichorionic-diamniotic.

Monozygotic twins result from one egg that splits, and depending on when the egg splits can result in several variations of a separate or shared amniotic sac or placenta. If the egg splits within 2-3 days after fertilization, the twins are dichorionic-diamniotic, each with its amniotic sac and placenta.  If the embryo splits between 3-8 days, the twins are monochorionic-diamniotic twins, so they each have their sac but share a placenta. If the embryo splits between 8-13 days, the twins are monochorionic-monoamniotic twins and share both a sac and a placenta. If the embryo splits after 13 days, then the twins are monochorionic-monoamniotic and are conjoined twins. 

Placenta Abnormalities

Before the widespread use of ultrasounds, few expecting mothers knew what position their placenta was in or if it was considered normal. When you have your anatomy scan ultrasound, usually between weeks 18-22, the ultrasound technician or care provider will examine your placenta. There are many variations in size and placement. Some of these abnormalities are simply variations that have no impact on your baby, and others can signal potential complications your care provider will want to monitor.

Placental Placement

The placement of your placenta is classified as anterior or posterior. Anterior means it is in the front of your uterus, closest to your belly. Posterior means it is attached to the back of your uterus, closer to your spine. It is thought the ideal position is posterior near the top of your uterus to allow your baby to move to an ideal position just before birth. If your placenta is located somewhere else, it does not mean your baby cannot move in the direction or position they need to. If you have an anterior placenta, you may not feel as much movement or as many kicks from your baby since the placenta can act as a cushion between your baby and your belly. It is also possible that an anterior placenta makes it more challenging to hear your baby’s heartbeat since the placenta is between the outside of your belly and your baby. In most cases, it makes no difference to your baby’s health if your placenta is anterior or posterior.

Low-lying Placenta and Placenta Previa

A low-lying placenta is defined as a placenta ending within 2 cm of the internal cervical opening but not covering it. If the placenta grows over the cervix, it is diagnosed as placenta previa. As the uterus expands, it can pull the placenta higher and away from the cervix, which resolves the situation. The later in pregnancy that placenta previa exists, the more likely it will be present at birth.

Some of the symptoms of placenta previa are bright red vaginal bleeding without pain, especially during the second half of your pregnancy, and some women also experience contractions. If you have placenta previa, your care provider will monitor your placenta’s position with an ultrasound throughout your pregnancy. You may be put on bed rest, and if the placenta has not moved enough by the time your baby is ready to make its entrance to the world, you will likely require a cesarean section. 

In a study of over 1,200 women with low-lying placentas, 98.4% had the low-lying placenta resolved.  89.9% of placentas cleared the cervix by 32 weeks, and 95.9% cleared by 36 weeks.  .6% of the total women had placenta previa or a low-lying placenta at or near term, and all had cesarean deliveries. In another study of 366 cases, 84% of complete placenta previas and 98% of marginal placenta previas, or low-lying placentas, resolved at a mean gestational age of 28.6 weeks.

Placenta Accreta Spectrum

Placenta accreta is a condition that occurs when your placenta grows too deeply into your uterine wall. This is a spectrum disorder classified by how deeply the placenta embeds into the uterus. Classifications include placenta increta, placenta percreta, and placenta accreta. The main risk of placenta accreta is hemorrhage because the placenta does not detach from the uterine wall after birth. This can be a severe complication and potentially life-threatening.

The rate of placenta accreta has been increasing in recent decades. In the 1970s and 1980s, this happened in 1 in 2,510 to 4,017 pregnancies. According to the American College of Obstetricians and Gynecologists, a study in 2016 found the instance to be 1 in 272 pregnancies. The increase is likely due to cesarean births, and the most common risk factor is a previous cesarean delivery. It is believed that a scar in the uterus contributes to the placenta not attaching normally. However, it is possible to develop placenta accreta in the absence of scarring in your uterus.

This is a diagnosis that is classified as high-risk. It is likely your doctor would recommend an early cesarean delivery before 36 weeks. ACOG recommends delivery takes place at a level III or IV maternal care facility. This would be a hospital that has the capabilities and the staff to handle complex obstetric complications. Since placenta accreta can be life-threatening, a hysterectomy is often recommended. For more information on placenta accreta and the guidelines, your doctor is likely following see this information from ACOG.

Variations in Shape

Placentas are typically a round or oval shape with the umbilical cord inserted around the middle. There are variations in shape and cord insertion that can occur. Some of these variations can affect the vascular structure of the placenta, which can make it less efficient.

Bilobed Placenta

A bilobed placenta has two roughly equal-sized lobes separated by a membrane. This variation occurs in 2-8% of pregnancies. A placenta with more than two lobes is called a multilobate placenta and is rarer. This diagnosis does not increase the risk of fetal anomalies. It can be associated with first-trimester bleeding, polyhydramnios, abruption, and retained placenta.

Succenturiate Placenta

A succenturiate placenta is a smaller variant of a bilobed placenta in which one or more accessory lobes develop apart from the main placental body. This variation occurs in 5% of pregnancies.

Circumvallate placenta

A circumvallate placenta occurs when the side facing the baby is too small and causes the membranes to fold back around the edges of the placenta. This can not always be detected by ultrasound and is often diagnosed after the placenta is delivered. Evidence shows this condition does increase risks for adverse outcomes like preterm delivery, placental abruption, emergency cesarean, small for gestational age, neonatal intensive care admission, and neonatal death.

Circummarginate placenta

A Circummarginate placenta is similar to a circumvallate placenta except that the edges of the placenta are flat. While this is a variation from normal, it does not increase risks to you or your baby.

Placenta Membranacea

Placenta membranacea is very rare and is estimated to affect 1 in 20,000-40,000 pregnancies. This condition occurs when the placenta develops as a thin structure wholly or partially covering the fetal membranes. This is associated with some adverse outcomes, but it is very rare.

Ring-shaped placenta

The ring-shaped placenta is a variant of the placenta membranacea. It is most often more of a horseshoe shape, can be a complete ring. This is associated with antepartum and postpartum bleeding and fetal growth restriction. This occurs in less than 1 in 6000 pregnancies.

Placenta Fenestrata

Placenta fenestrata is a rare condition when a portion of the placenta is thinning, and in very rare cases, there is a hole in the placenta. This may increase the risk for a retained placenta.

Abnormal Placental Cord Insertion

In most placentas, the umbilical cord is inserted near the center. Marginal cord insertion (battledore placenta) occurs when the umbilical cord is inserted on the side. If the cord is inserted into the membrane and the vessels are partially exposed, it is considered a velamentous cord insertion.

A population-based study of 634,741 pregnancies found abnormal placental cord insertion incidence in single pregnancies was 7.8% (1.5% velamentous, 6.3% marginal). This was more common with twins at 16.9% (6% velamentous, 10.9% marginal). Velamentous and marginal insertion were associated with an increased risk of adverse outcomes, including placenta previa, placental abruption, pre-eclampsia, preterm birth, low Apgar score, transfer to NICU, low birth weight, and malformations. A systematic review and meta-analysis found abnormal placental cord insertion associated with an increased risk of emergency cesarean delivery.

Signs and Symptoms

Many of these variations in your placenta or umbilical cord are found during an ultrasound and most commonly during the anatomy scan ultrasound between weeks 18-22. Most expecting mothers who have a placenta or umbilical cord that differ from normal have no symptoms. Some of these conditions can cause vaginal bleeding, which you should contact your doctor or midwife about right away.

What to Do if You are Diagnosed with an Abnormal Placenta

The first thing you should not do is panic. In a perfect world, you hear that everything looks ideal after an ultrasound exam. Keep in mind that the anatomy scan ultrasound has become routine because it is an opportunity to learn about conditions that could be treated or affect your pregnancy or birth. Some of these diagnoses, like an anterior placenta, have little clinical significance and rarely affect your pregnancy. Other findings, like placenta accreta, allow your care provider to prepare for all scenarios and ensure that you have the best care available to prevent adverse outcomes.

Many of these conditions may prompt your care provider to recommend more frequent ultrasounds or monitoring. This can alert you, and your care team in the event intervention is needed or confirm that a condition like a placenta previa has resolved itself over time.

It can be challenging to hear the news that anything in your pregnancy is less than perfect. Due to COVID-19 and changes in policies, you may be in these appointments alone. If you have the opportunity to bring your partner or a friend to appointments, please do. If that is not an option, you can always get your partner or a support person on a video chat or the phone, so you feel you have support from someone in addition to your care provider.

Talking to Your Doctor or Midwife

Please discuss your diagnosis in-depth with your doctor or midwife so you can fully understand what it means, how it may impact your pregnancy and birth, and what your options are. Often fear comes from the unknown. Please ask questions and advocate for yourself to get the answers you need to feel more comfortable with the following steps or options. You may also consider talking to a specialist, like a maternal-fetal medicine doctor or a doctor who specializes in high-risk pregnancies if that applies to your situation.

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