Post-term Pregnancy Causes and Risks

Post-term pregnancy is a pregnancy that lasts longer than 42 weeks.  That is two weeks past the normal 40-week gestation period. Most women deliver their babies between 37 and 40 weeks, which is considered safe and “normal”. Post-term pregnancy is linked to both fetal and maternal health complications. Therefore, doctors usually do everything they can to ensure that an infant is delivered as close to the due date as possible.  In some cases that means inducing labor, which comes with its own set of risks.

There are a few things relevant to post-term pregnancy that women should be aware of:

• Some women carry pregnancies longer than others.
• Healthcare providers don’t know why some pregnancies last longer than others.
• During pregnancy, you may need to be monitored for signs of complications.
• If it becomes dangerous for the pregnancy to continue, doctors may induce labor or schedule a cesarean delivery (c-section).

Post-term Pregnancy Causes

In many cases, post-term pregnancy is a matter of miscalculating the date of conception.  Yet, physicians should always perform an ultrasound during the first half of the pregnancy to promote accuracy.  Although there is never a guarantee of an infant’s due date, an early ultrasound will give a better idea of when parents can expect their new arrival.

Healthcare experts don’t fully understand why some pregnancies last so long. Aside from miscalculating the date, experts believe there are some risk factors that may contribute to post-term pregnancy:

• Previous post-term pregnancies
• Maternal obesity
• Sulfatase deficiency in the placenta
• Central nervous system abnormalities
• Anencephaly

What are the Risks of a Post-term Pregnancy?

According to the American Academy of Family Physicians (AAFP), there are numerous dangerous health risks associated with a prolonged pregnancy, including:

Fetal Macrosomia

Fetal macrosomia refers to an infant who is over 8 pounds, 13 ounces at birth.  This means that the baby is unusually large for their gestational age. An unusually large baby can cause problems during labor and delivery. It can also lead to childhood diabetes, obesity and metabolic syndrome.  Mothers are also at risk when delivering a large baby. They may suffer uterine rupture, genital lacerations and excessive bleeding after delivery.

It is important that doctors choose the best labor and delivery options for fetal macrosomia. If labor is prolonged or the baby becomes stuck in the birth canal, they are at risk for serious birth injuries like brachial plexus injury, oxygen deprivation, nerve damage and brain damage. A post-term pregnancy may make the risk of a large baby greater because the baby continues to grow in the womb after the due date.

Placental Insufficiency

Placental insufficiency, also known as uteroplacental vascular insufficiency, occurs when the placenta fails to deliver adequate oxygen and nutrients to the infant.  This can occur due to a small placenta, or because the placenta is not functioning properly. The placenta provides blood, oxygen and nutrients to the baby via the umbilical cord. After 37 weeks of pregnancy, the placenta reaches its maximum size and its functions begin to reduce. By 41 weeks, functions are greatly reduced. The longer an infant goes without proper nutrition and oxygen, the more at risk they are for a host of health problems. 

Some possible health problems include oxygen deprivation at birth which can lead to cerebral palsy and learning disorders.  Since the placental cord may compress in a post-term pregnancy, there is a heightened risk of placental insufficiency. 

Meconium Aspiration

Meconium aspiration is marked by an infant breathing in amniotic fluid and meconium (newborn feces) during labor and delivery.  Generally, infants pass meconium (their first bowel movement) in the first few days after birth. However, infants who are born post-term are more likely to have a bowel movement while still in utero.  This is what causes the condition known as meconium aspiration syndrome. You may also hear the term meconium stained amniotic fluid.

Meconium aspiration is extremely dangerous. Meconium is thick and sticky, and can block the baby’s airways. It can also lead to oxygen deprivation, lung inflammation and lung infection.  Although rare, it can also lead to persistent pulmonary hypertension of the newborn (PPHN) and permanent brain damage.

Risks to Mothers

Mothers are also at risk for developing dangerous medical issues if their pregnancy continues past 40 weeks. Mothers are at risk for complications during labor and delivery, as well as complications that could affect their health after delivery. The most common risks to mothers include postpartum hemorrhaging, bacterial infections, perineum injuries and a chance of requiring a cesarean delivery (c-section) surgery.

Complications of Post-Term Pregnancy

When a pregnancy goes longer than 40 weeks, there is a higher risk of certain complications. When a pregnancy goes post-term, women are at a greater risk of:

• Longer labor
• Vaginal tearing
• Forceps or vacuum extraction during labor
• Cesarean delivery
• Infection
• Wound complications
• Placenta problems
• Low levels of amniotic fluid

There are also risks to the unborn and newborn baby, such as:

• Meconium aspiration syndrome (breathing in amniotic fluid and meconium)
• Low birth weight for gestational age
• Low blood sugar (hypoglycemia)
• Stillbirth

How Doctors Diagnose a Post-Term Pregnancy

The only way to really diagnose a post-term pregnancy is to calculate when the due date is, or should be. To do this, your doctor may do maternal and fetal testing, such as the following:

• Measure the size of your uterus
• Estimate the baby’s gestational age
• Compare the size of your uterus with earlier measurements
• Note when the first fetal heartbeat was detected
• Note when you first reported feeling fetal movement
• Perform an ultrasound

Your doctor may also do some additional tests to make sure the baby is healthy. These tests include:

• Measuring amniotic fluid volume
• Performing additional ultrasounds
• Assessing how the baby’s heart rate reacts to activity

Options for Managing a Post-term Pregnancy

Treatment is imperative for post-term pregnancy and if done properly, may help prevent many of the aforementioned risks.  Typical treatment options may include:

Antenatal Fetal Monitoring

An infant may be monitored closely once the due date passes in order to detect any signs of distress.  The AAFP doesn’t recommend antenatal fetal monitoring until the 42nd week pregnancy. Antenatal fetal monitoring, also called antepartum fetal surveillance, is a type of monitoring that is designed to reduce the risk of stillbirth. This type of detailed monitoring includes monitoring fetal heart rate, umbilical Doppler velocimetry and using real-time fetal ultrasound. Antenatal fetal monitoring is especially important in cases where the mother has a pre-existing condition, such as gestational diabetes, or when there are fetal issues like intrauterine growth restriction (IUGR).

Contraction Stress Test

A contraction stress test will provide Oxytocin to the mother in an attempt to start contractions.  Depending on how the baby’s heart rate responds to the contractions determines next steps. Contract stress tests (CST) are not as common as they used to be, but they can be beneficial in helping assess how labor will progress. During a CST, you will wear a belt around your belly that will measure your baby’s heart rate. Another belt will measure contractions. Doctors generally administer Oxytocin through an IV to stimulate contractions. The belts monitor the contractions and your baby’s heart rate, and are careful to assess for possible labor induction.

If your baby’s heart rate drops during CST, it could be a sign that labor will be difficult or dangerous. If that happens, your doctor will likely order additional tests, or they may decide that it is safer to proceed with early delivery.

Biophysical Profile

A biophysical profile (BPP) is a nonstress test that will determine an infant’s overall physical score in regards to movement, breathing, fetal tone and the volume of amniotic fluid. A nonstress test and an ultrasound are done, and a score is given based on the criteria. This is a noninvasive way to estimate pregnancy outcomes and assess the health of the baby. The tests are simple and can be done in your doctor’s office in about 30 minutes.

Scoring for a biophysical profile range from 0 to 10. There are five categories that each can receive a score of 0-2. These categories are fetal:

• Heart rate
• Breathing
• Movement
• Muscle tone
• Amniotic fluid volume

Once the test is done, the doctor will add up the score. A score of 8-10 is considered reassuring, or positive. A low score on a biophysical profile – 6 or less – could indicate complications or health concerns. If that happens, your doctor will likely order additional tests. If there is reason for concern about the baby’s health, immediate or early delivery may be necessary. If at any time the doctor feels like the baby is in distress, it may be necessary for you to delivery early, regardless of the biophysical score.

Maternal and fetal testing are important if your pregnancy is high-risk or if your doctor is worried about possible complications.

Labor Induction

It’s often difficult to determine the best time to induce labor, but if the results of the previously mentioned treatment options indicate fetal distress, physicians will normally induce labor.  Labor induction can include medication applied to the cervix that promotes contractions or an IV infusion of oxytocin, called Pitocin.

Doctors use what is called the Bishop score to estimate how successful labor induction will be. This score evaluates the fetal position, dilation, consistency and cervical effacement. If the score is greater than six, induction is favorable. If it is less than five, doctors will assess if induction or a cesarean delivery is the best option.

It’s important to note that labor induction comes with its own health risks.  These include low fetal heart rate, excessive maternal bleeding and umbilical cord problems. If the cervix is ripe for induction, then cesarean section for failure to progress is less likely. However, in most cases, labor induction greatly outweighs the risks of allowing the pregnancy to continue if the infant is in distress.

When to Call Your Doctor

Most women have weekly prenatal appointments once they reach 36 weeks of pregnancy. Make sure you keep these appointments and update your doctor on any changes you notice in your pregnancy. If your pregnancy continues past 41 weeks, you and your doctor will discuss your options. Depending on your health, and to ensure the health of your baby, you may decide to schedule a c-section.

In between your appointments, call your doctor if you notice any of the following:

• Your water breaks (membranes rupture and amniotic fluid rushes out)
• You have vaginal bleeding
• There is a notable change in fetal movement. The baby should move at least 6 times per hour while you are resting on your left side. 

 As we mentioned before, it is important to carefully monitor your baby’s health to ensure that continuing a post-term pregnancy is not more risky than inducing labor or scheduling a c-section. A prolonged pregnancy doesn’t guarantee that you will have complications, but it does increase certain risks.

How to Improve Your Pregnancy Prognosis

If you have concerns about your pregnancy, labor or delivery, talk to your doctor. Never be afraid to ask questions and address your concerns. Keep all prenatal appointments and use this time to consult with your doctor and make and/or maintain a plan for your pregnancy, labor and delivery – or a post-term pregnancy. Here are some things to keep in mind as you attend prenatal appointments and work toward a healthy pregnancy:

• Be aware of the reason for the visit and any scheduled tests.
• Write down any questions you have before the visit.
• Bring someone to the visit with you to help you remember what the doctor said.
• Write down any information relevant to your amniotic fluid, the baby’s size and how your pregnancy is progressing.
• If you got any new diagnosis, medicines or test results, make sure you write those down after the visit.
• Write down any new instructions the doctor gave you.
• Explore any medications you are prescribed, and make sure you understand why you are taking it and if there are possible side effects.
• If your doctor orders any maternal or fetal testing, make sure you understand what they are and why you need them.
• If you hear terms like “low blood sugar” or “gestational diabetes” make sure you understand what they mean.
• Make sure you keep any follow up appointments.
• If your doctor discusses a c-section, ask questions about possible wound complications, healing time and side effects.

Where to Get More Information

Your best source of information about your health and your pregnancy are your healthcare providers. However, if you are looking for general information about post-term pregnancy and potential risks, Birth Injury Guide can help. Take a look at the following articles to learn more:

• Fetal Macrosomia
• Maternal Infections
• Infant Brain Damage
• Shoulder Dystocia

If you believe that you have received substandard medical care or treatment related to a post-term pregnancy, Birth Injury Guide can also help you explore your legal rights and determine if you are a victim of medical negligence. If so, you may have a claim for compensation for your injuries and related losses.

Page Medically Reviewed By Kim Langdon, M.D.

Kimberly Langdon, M.D. is a retired board-certified OB/GYN with 19 years of clinical experience. She currently works as a medical writer and featured healthcare expert. She is a regular medical reviewer for Birth Injury Guide.

Read Bio

Written By Sam Uribe

Sam Uribe is a researcher and writer. She lends her expertise to the team at Birth Injury Guide to provide up-to-date and relevant content that clients can count on.