Macrosomia is the medical term for a baby that has a significantly higher birth weight than the average for newborns. The term is also known by the common term big baby syndrome. The other common term that overlaps with macrosomia is large for gestational age. Macrosomia is usually defined as a birth weight of more than 8 pounds 13 ounces (4 kilograms). It may also be defined by the gestational age which in turn is influenced partly by the gender of the newborn and ethnicity. Since male newborn babies are typically larger and heavier than females, macrosomia is more likely in male newborns. Overall, macrosomic babies are significantly larger compared to average infants.
Macrosomia is dangerous for both the baby and the mother. It increases the risk of birth trauma during a vaginal delivery. The risk is greater if the birth weight reaches 9 pounds 15 ounces. There is also a greater chance of a difficult labor and stillbirth. Cesarean delivery is strongly recommended in cases of macrosomia. Macrosomia also increases the risk of health problems (diabetes mellitus) in babies after birth.
Macrosomia is reported in 1 to 10% of babies born worldwide. Apart from the host of diseases that may influence the baby’s birth weight, there is an increased risk for overweight women. As obesity reaches epidemic proportions globally, the incidence of macrosomia will undoubtedly rise as well. Around 10% of newborns weigh more than 8 pounds, 13 ounces, whereas in 1% of newborns, the birth weight reaches 9 pounds 15 ounces.
The frequency of macrosomia is higher in :
- male infants
- newborns of Hispanic origin
- prolonged pregnancies
Macrosomia is not simply a matter of a chubby newborn. It is called big baby syndrome because the baby is literally larger in size. Various hormones influence growth and development, not only in fetal life but also in childhood and even through adulthood to some extent. The quantity of these hormones in the bloodstream determines the rate of growth which can ultimately affect size. The situation is no different for the fetus.
In some instances, macrosomia is a result of an overgrowth disorder known as Perlman syndrome. This is a serious and severe genetic disorder that is associated with neonatal death. Another rare condition where macrosomia is seen is Beckwith-Wiedemann syndrome which is not a genetic disorder.
Hormones that affect growth
If the amount of hormones that influence growth is higher than normal, then the fetus will grow to a larger size. These hormones include growth hormone, insulin and other growth hormone-like and insulin-like compounds. This is the main reason why big baby syndrome is more likely in mothers who are diabetic or have insulin resistance. There may also be genetic factors at play as well as other diseases which impact on hormone levels.
Mother and baby
Many conditions can lead to the development of macrosomia. In pregnant women, poorly controlled diabetes, excessive weight gain during pregnancy, high blood sugar (hyperglycemia) and obesity are associated with macrosomia. In the fetus, high blood sugar can stimulate the production of hormones that promote fat accumulation and fetal growth. Prolonged pregnancies (more than 42 weeks) also result in larger birth weights in babies.
Possible signs and symptoms may include:
- A body mass index (BMI) value of 30 or more in pregnant women indicates obesity and is associated with larger infants at delivery.
- Large fundal height (the size of the uterus measured from the top of the uterus to the pubic bone) can be an indication of fetal macrosomia in some cases.
- Excessive amount of the fluid surrounding the fetus (amniotic fluid) can indicate more urine output by a large fetus in some cases.
However, both large fundal height and more amniotic fluid can be indicative of other conditions. Therefore these factors alone cannot be used to accurately predict macrosomia.
The complications associated with macrosomia can be grouped according to the impact on the mother or the baby.
Risk to Mother
- Labor difficulties where the fetus may be stuck in the birth canal thereby requiring an operative vaginal delivery or cesarean section.
- During labor, macrosomic fetuses can injure the birth canal by tearing vaginal tissues and the muscles between the anus and the vagina.
- If the muscles of the uterus do not contract properly after delivery (uterine atony), it can lead to potentially serious bleeding.
- In rare cases, uterus might tear open along the scar line formed during a previous C-section or other uterine surgeries (uterine rupture). This can lead to life-threatening complications.
Risk to Baby
- The newborn may develop impaired glucose tolerance (higher blood sugar levels).
- Increased birth weight may further develop into childhood obesity.
- Macrosomia may result in metabolic syndrome (characterized by high blood sugar, increased blood pressure, abnormal cholesterol levels, and excess body fat), which increases the risk of stroke, heart disease, and diabetes.
- Newborns may suffer from brachial plexus injuries (damage to the nerves originating near the neck and shoulder).
- Low blood sugar levels, hematological disturbances (like excessive red blood cells) and electrolyte disturbances (such as abnormal levels of sodium, potassium, and calcium) are commonly seen in macrosomic newborns.
Genetic factors, maternal conditions (like obesity or diabetes), or certain medical conditions can cause a larger birth weight in macrosomic babies. However, in many cases the causes may remain unexplained. Factors that can contribute to excessive weight gain and fetal growth may include the following:
- Poorly controlled diabetes mellitus is the greatest risk factor for developing macrosomia. Increased glucose levels in the mother can result in stimulation of fat accumulation and fetal macrosomia.
- Obesity, increased pre-pregnancy weight, and excessive weight gain during pregnancy can also lead to excessive growth in some cases.
- Genetic factors (like height and weight of the parents) also contribute to fetal size.
Big baby syndrome is more likely in light of the following risk factors.
- Diabetic mothers and gestational diabetes
- Obese women
- Advanced age during pregnancy
- Excessive weight gain during pregnancy
- Genetic, racial, and ethnic factors
- Overdue pregnancy (postterm)
- Multiple births
- History of macrosomia
- Excessive amniotic fluid
In many cases, however, none of the risk factors can explain the variations in the birth weight. Very rarely, some medical conditions may affect fetal growth. A prenatal diagnostic test for these conditions is therefore recommended in these cases.
Increased birth weight or macrosomia cannot be accurately predicted. Even with estimates, there are many known and unknown factors that can influence the difference between the fetal weight compared to the actual birth weight. It can be confirmed only after delivery. There is also a slight drop in weight 24 hours after birth. Overall, it is difficult to detect and diagnose macrosomia during pregnancy. The signs and symptoms mentioned above may be an indicator of macrosomia. Ultimately however, the condition is diagnosed at birth.
Some of the diagnostic investigations that may be useful in possibly indicating macrosomia includes :
- Ultrasound to provide estimated fetal weight
- Abdominal circumference measurements up to 2 weeks before delivery
However, both these measures may only be useful just before giving birth. By then the processes that contribute to macrosomia have already been underway and cannot be reversed.
Certain measures may need to be considered to avoid complications during childbirth. Prevention is the key in dealing with macrosomia.
To avoid unnecessary complications, a cesarean section may be recommended for delivery. However, even cesarean deliveries are complicated by macrosomia and place the mother at risk in terms of :
- damage to adjacent organs ( uterus, bladder, ovaries, fallopian tubes, ureter, and intestines)
- complications related to the use of anesthesia
Cesarean section may be recommended in cases of:
- Maternal diabetes, either before pregnancy or during pregnancy (gestational diabetes)
- Estimated fetus weight of 9 pounds 15 ounces or more
- Estimated fetus weight of 11 pounds or more in non-diabetic mothers
- Previous history of shoulder dystocia (newborn’s shoulder getting stuck behind mother’s pelvic bone) in diabetic or obese pregnant women
Inducing labor does not reduce the complications associated with fetal macrosomia and is not generally recommended.
Fetal macrosomia cannot be prevented but a healthy pregnancy can be promoted by:
- Avoiding obesity and maintaining proper body weight before pregnancy
- Gaining a healthy amount of weight during pregnancy
- Managing diabetes mellitusd
- Indulging in physical activities daily
- Diet control with insulin therapy (in diabetic mothers)
- Consultation with a perinatologist (a specialist in maternal-fetal medicine) to obtain better control
Article reviewed by Dr. Greg. Last updated on July 27, 2012