Fetal macrosomia is the medical term for when a baby is born much larger than the average size for their gestational age. Gestational age refers to the amount of time the fetus has been in the uterus.
Babies without fetal macrosomia typically weigh between 5 pounds (lb) 8 ounces (oz) and 8 lb 13 oz. Babies with fetal macrosomia weigh more than 8 lb 13 oz.
The increased size and birth weight of babies with fetal macrosomia can cause complications during delivery that may affect the health of both the birthing parent and the fetus. Babies born with fetal macrosomia also have a higher likelihood of experiencing certain health problems later in life.
This article describes what fetal macrosomia is, including its causes and risk factors. It also outlines some of the possible complications associated with delivering a baby with fetal macrosomia and provides information on diagnosing and preventing fetal macrosomia.
Fetal macrosomia is a condition in which a newborn baby is much larger than average for their gestational age. Gestational age refers to the amount of time the fetus has been in the uterus.
The term macrosomia means “big body.” It derives from the Greek words “macro,” meaning “big,” and “soma,” meaning “body.”
Babies without fetal macrosomia typically weigh between 5 lb 8 oz and 8 lb 13 oz. Babies with fetal macrosomia weigh over 8 lb 13 oz, which is equivalent to around
Fetal macrosomia is associated with potentially life threatening complications for both the baby and the birthing parent.
There are various potential causes of fetal macrosomia. Some arise from the birthing parent, and others arise within the fetus.
The sections below look at some
Causes of fetal macrosomia associated with the birthing parent include:
- Diabetes: If the birthing parent has diabetes, the fetus may overuse glucose, and this may trigger unusual growth. Types of diabetes that may cause fetal macrosomia include:
- insulin-dependent diabetes
- chemically induced diabetes
- gestational diabetes
- Obesity: According to one
2021 article, obesity in the birthing parent is associated with a four to 12 times increase in the risk of fetal macrosomia. This may be due to increased insulin resistance in the fetus.
- Multiparity: This is the medical term for when a person has given birth more than once. Experts do not consider multiparity a major risk factor for fetal macrosomia. However, it may contribute to diabetes and obesity, which are more closely linked to the condition.
- Previously giving birth to a baby with fetal macrosomia: People who have previously delivered a baby with fetal macrosomia have a five to 10 times higher risk of delivering another baby with fetal macrosomia.
- Prolonged gestation: A person who is pregnant for longer than 42 weeks is more likely to have a baby with fetal macrosomia. This is because the fetus absorbs a constant supply of oxygen-rich blood and nutrients that support continued growth.
Causes of fetal macrosomia associated with the fetus
- Beckwith-Wiedemann syndrome: This is an overgrowth condition that affects specific chromosomes.
- Sotos syndrome: This is a rare genetic disorder that results from a mutation in the NSD1 gene. The condition causes excessive physical growth during the first few years of life.
- Fragile X syndrome: This is an inherited genetic condition that can affect intellectual and developmental faculties.
Fetal macrosomia is more common among babies assigned male at birth than babies assigned female at birth. This may be because the former are typically heavier than the latter.
A person is more likely to have a baby with fetal macrosomia if they:
- have diabetes before pregnancy or develop diabetes during pregnancy
- have high blood pressure during pregnancy
- gain a lot of weight during pregnancy
- have obesity
- are over 35 years of age
- are more than 2 weeks past their due date
- have previously had a baby with fetal macrosomia
- are expecting their baby to be assigned male at birth
Fetal macrosomia can
During delivery, fetal macrosomia can increase the risks of the following:
- longer labor
- the need to use forceps or a vacuum
- cesarean delivery
- the fetus’s shoulder getting stuck in the birth canal
- fractures to the fetus’s collarbone or other bones
- respiratory issues in the fetus due to lack of oxygen
Risks for the birthing parent
Giving birth to a baby with fetal macrosomia may increase the risks of the following occurrences during labor:
- Uterine rupture: This is a complication in which the uterus tears, causing the fetus to slip into the birthing parent’s abdomen. This complication can be life threatening for both the parent and the fetus.
- Vaginal injury: During birth, a fetus that is larger than usual can tear the birthing parent’s vagina and perineal muscles. These muscles sit between the vagina and the anus.
- Bleeding: Due to the fetus’s large size, the muscles of the uterus may not contract as they normally would after delivery. This can cause excessive bleeding.
Risks for the baby
Babies born with fetal macrosomia are at increased risk of trauma and death compared with babies who are a typical weight at birth.
The fetus may experience the following trauma during childbirth:
- becoming stuck in the birth canal
- sustaining fractures to the collarbone or other bones
- developing breathing issues due to insufficient oxygen
A baby born with fetal macrosomia may also be at increased risk of the following conditions in later life:
- Labor induction: Experts suggest that pregnant people with diabetes have the option to receive elective birth by induction of labor after 38 weeks of gestation.
- Cesarean delivery: Some experts recommend cesarean delivery for people with diabetes and those who are due to deliver a baby with an estimated fetal weight above 9 lb.
A person who is pregnant and worried about delivering a baby with fetal macrosomia should talk with a doctor or midwife.
They will take a full medical history, including any history of previous pregnancies. They will also check the size of the fetus throughout the pregnancy using the following methods:
- Ultrasound: This method uses ultrasound waves to view the fetus and help estimate its size within the uterus.
- Fundus measurement: A doctor may measure the length of the fundus, which is the area from the top of the uterus to the pubic bone. A fundus that is larger than usual may indicate fetal macrosomia.
- Amniotic fluid level: An above-average amount of amniotic fluid indicates that the fetus may be producing excessive urine. Larger fetuses produce greater levels of urine.
- Nonstress test: This test measures the fetus’s heartbeat when they move.
- Biophysical profile: This profile combines information from the ultrasound and nonstress tests to assess the fetus’s breathing, movements, and amniotic fluid level.
The most effective way to manage fetal macrosomia is to prevent its occurrence. Two of the most significant factors associated with fetal macrosomia that a person may be able to control are obesity and gestational diabetes.
Losing excess weight or maintaining a moderate weight before and throughout pregnancy reduces the risk of gestational diabetes. This, in turn, reduces the risk of delivering a baby with fetal macrosomia.
People who already have diabetes should ensure that they maintain control of their blood glucose levels throughout their pregnancy. Doing so will help reduce the risk of fetal macrosomia.
Someone who discovers that they are at risk of giving birth to a baby with fetal macrosomia may experience anxiety about the labor, their own health, and the health of the fetus.
This anxiety can cause undue stress to the birthing parent and the fetus. A doctor or midwife may offer recommendations for coping with the added stress and anxiety, such as psychological therapies or relaxation therapy.
A healthcare professional will also be able to discuss different birthing procedures that may help reduce the risk of complications associated with fetal macrosomia.
Fetal macrosomia is the medical term for when a baby weighs more than 8 lb 13 oz at birth, whether they are born early, on time, or late. The condition may occur as a result of genetic conditions affecting the fetus or due to underlying health conditions, such as obesity or diabetes, in the birthing parent.
Fetal macrosomia may lead to complications for both the birthing parent and the fetus.
Complications that may affect the birthing parent include uterine or vaginal tearing and excessive bleeding following labor.
Complications that may affect the fetus include becoming stuck in the birth canal, sustaining bone fractures, and experiencing breathing difficulties. Babies born with fetal macrosomia may also be at increased risk of medical complications such as obesity, hypertension, and insulin resistance later in life.
A person who is concerned about delivering a baby with fetal macrosomia should talk with a doctor or midwife for advice. People can reduce their risk of delivering a baby with fetal macrosomia by maintaining a moderate weight and controlling their blood glucose levels before and throughout pregnancy.