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A few weeks ago, we discussed the risks of giving birth to large babies and what to do when babies are born large.
However, I have received new questions about large babies; LGA (large for gestational age). The reader is wondering if we can further explain why babies become large and if expectant mothers can do anything about it.
First, let me remind you of the definitions that are used (sometimes a bit loosely).
Large For Gestational Age (LGA)
The general definition of LGA varies depending on where the child is born in the world. Therefore, there is no universal definition, and different countries use different criteria.
Being large for gestational age (LGA) means a newborn baby weighs more than usual relative to its age (the number of gestational weeks).
Infants can be considered large for gestational age if they weigh more than 9 out of 10 babies (90th percentile) or more than 97 out of 100 babies (97th percentile) at the same gestational age.
The definition in Sweden is
+2 standard deviations or approximately the 97th percentile, which means among the approximately 3% largest babies for their gestational length.
Sometimes, the term "Macrosomia" is also used, which means fetal weight ≥ 4500 g, regardless of gestational length.
Large babies - Why is this a significant issue?
Towards the end of pregnancy, the fetus's weight increases by an average of 200 g per week. If the child has accelerated growth, it means that the child is growing at a rate that exceeds the average increase.
The rule is that accelerated growth should be confirmed with two ultrasound measurements taken two to four weeks apart. This is because we know that assessing large babies has relatively low accuracy, lower than with babies of normal weight.
We know that there is a margin of error in weight estimation of +/- 8%. Knowledge suggests that it is more challenging to measure large babies, so the margin of error can be even larger.
So, why is this a significant issue?
Firstly, we know that giving birth to a large baby carries risks for both the woman and the baby. That's why knowledge is essential. Secondly, the diagnosis is as uncertain as it is, and we don't really know how to best manage these pregnancies.
Why do some children become large?
It can be observed that there are many factors influencing fetal growth, including genetic, nutritional, environmental, and also the function of the placenta.
The likelihood of giving birth to a large baby increases if you have:
Previously given birth to a large baby
As an expectant mother, have a high body mass index (BMI > 30)
Experienced a weight gain of more than 15 kg during pregnancy
Overdue pregnancy (the fetus gains approximately 200g per week)
Maternal diabetes (Type 1, but also Gestational Diabetes Mellitus - GDM)
Maternal height
Some of these factors are therefore influenced by the woman.
A large international study published in Nature Genetics in 2019
The study showed that a child's birth weight is influenced by both the mother's and the child's genetics and the interplay between them.
The study identified 190 connections between genetics and birth weight, two-thirds of which were previously unknown. The conclusion was that the child's own genes had a significant and direct impact on birth weight.
However, the interaction between mother and child is complex.
Approximately one-quarter of the identified genetics that influenced birth weight came from factors in the child's environment, such as the availability of glucose during pregnancy, which was, in turn, directly influenced by the mother.
Understanding the factors that affect birth weight is important because babies born with very high or very low birth weights have poorer survival and a higher risk of developing metabolic diseases later in life.
What the Swedish Pediatric Association Writes
In Sweden, the average birth length and weight are approximately 50 cm / 3.5 kg with a variation of 2 cm / 0.5 kg. Boys are about 7 mm longer than girls and 135 g heavier.
The length of pregnancy affects birth size. Growth towards the end of pregnancy corresponds to approximately 1 cm and 150-200 g per week. However, in cases of overdue pregnancy, intrauterine growth is slowed down.
Statistically, birth size is correlated with the mother's size - big mothers give birth to big babies, and small mothers give birth to small babies. However, in some families, it's common for children to be born large or small regardless of the mother's size.
A few anecdotes I can share, including one about a colleague of mine at the clinic:
She has given birth to four children, all vaginally, and all weighing over 5 kg. She herself is of average height and weight.
"I simply give birth to big babies," she says. "It's just how it is. My sister also gives birth to big babies."
The largest baby I've delivered in my career weighed 6.2 kg and was 58 cm long. This baby had an unstable fetal position(!), which meant it sometimes had its head down and sometimes lay across.
This eventually led to us performing a cesarean section. It was truly a giant baby that we delivered!
I wonder if it would have made it out the normal way if we hadn't done anything. The mother said that this was her third child. The first two both weighed over 5 kg and were delivered vaginally.
The proportion of babies with high birth weight has increased in recent decades, which can be partly attributed to the increase in the number of mothers with high BMI and the decrease in the frequency of smoking during pregnancy.
The average birth weight with maternal BMI >30 has increased by about 180 g compared to BMI <25.
Insulin Resistance
Insulin resistance is common in obesity and is accentuated during pregnancy due to the influence of hormones from the placenta.
The fetus is then exposed to the maternal increases in glucose and lipids found in the woman's blood, which can lead to increased fetal growth.
Since LGA is correlated with overweight/obesity and metabolic disease in adulthood, the increasing proportion of babies born LGA is likely to have consequences for the health of future generations.
As previously mentioned, the percentage of mothers who smoke has drastically decreased in Sweden. In 1983, 31% smoked during pregnancy, compared to 7% in 2007.
This also contributes to larger babies.
Big Baby Trial
An English RCT (Randomized Controlled Trial) study closed in late 2022 (the results have not yet been published).
Here, the aim was to study the proportion of large babies that experienced shoulder dystocia (where one of the baby's shoulders becomes stuck behind the mother's pubic bone after the baby's head has been delivered) and the risk of long-term complications such as brachial plexus injuries.
Shoulder dystocia occurs more frequently in larger babies, but there is uncertainty about how often this actually happens. It's sometimes estimated to occur in 1 out of 25 deliveries.
It has been suggested that inducing labor in larger babies a few weeks before the due date might result in smaller babies, reducing the risk of shoulder dystocia and lasting injuries to the baby.
Currently, it is unclear whether it is better for women with large babies to have their labor induced or to wait for spontaneous labor to start. To answer this question, this clinical trial is needed.
How was the study conducted?
In this study, 4,000 pregnant women were observed, and their ultrasound scans indicated that their babies were larger than expected.
With the woman's consent, they were then randomly assigned to either undergo induction at 38 weeks or to wait for spontaneous labor to commence.
There will also be a parallel cohort study of women who declined randomization (for example, if they wanted a cesarean section) but agreed to provide information on the outcome of their delivery.
This study will hopefully help women, obstetricians, and midwives better determine the best approach for managing pregnancies with large babies.
As you can imagine, this is not a straightforward matter.
I'm eagerly awaiting the results.
/Doctor Eva
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