Most pregnant mamas will, at some point, question the part genetics might play in the birth weight of their baby.
Babies come in all shapes and sizes. Fetal growth is a factor all care providers must consider, to ensure babies are growing and developing normally.
Normal fetal growth is dependent on the successful exchange of nutrients between the mother and the baby, via the placenta.
If this important exchange is hampered, or impaired, in any way, it can cause complications for the baby’s growth.
But there is one factor that can’t be changed, and one which might have the final say on how much a baby weighs at birth – and that’s genetics.
What Factors Can Affect Your Baby’s Weight?
Your baby’s birth weight is determined by a number of factors. Some of these factors are genetic and can’t be controlled, and others are environmental, meaning we can control them, up to a point.
- Birth order: the first baby tends to weigh less than younger siblings
- Gender: boys usually weigh more at birth than girls
- Your pre-pregnancy weight: women who are obese or overweight tend to give birth to larger babies
- History of diabetes or gestational diabetes: high blood sugar can cause your baby to gain weight
- Previous history of large babies: if you have already given birth to a baby weighing more than eight pounds, you’re more likely to have another of a similar size
- Your habits: if you smoke, take drugs, or drink excessive amounts of caffeine, your baby’s growth could be restricted
- Baby’s health: if your baby has a condition which can affect growth, she might be smaller at birth.
- Genes: you or your partner might come from a long line of large, or small, babies.
How Does Genetics Affect Birth Weight?
Each parent provides 23 chromosomes to a developing baby. When sperm and egg meet, they join to form the 46 chromosomes of a single cell. This cell divides rapidly until it becomes the 100 trillion cells that are your baby.
It would make sense to think that both maternal and paternal genes influence the size of your baby at birth, but this has turned out to be more complicated than we previously believed.
We already know there are several factors that can influence a baby’s weight, but research into a group of growth genes has shed new light on how growth can be affected from the very early stages of pregnancy.
For most genes, we inherit two working copies – one from mother and one from father. Expression of this gene comes from both parents. A very small number of genes, however, are imprinted, meaning expression comes from only one copy of the gene, and is determined by the parent who contributed the gene.
If the paternal gene is expressed, the maternal one is imprinted (silenced), or vice versa. The hormone, insulin-like growth factor 2 (IGF2), is expressed only from the gene inherited from the father, and silences or imprints the gene from the mother.
A study led by the UCL Institute of Child Health (ICH) looked at the connection between birth weight and the paternally expressed IGF2.
In early gestation, IGF2 is a key hormone that regulates growth. The researchers studied samples of IGF2 during the first trimester of pregnancy, and looked for correlations between the gene’s expression and the birth weight of the baby.
Expression of the paternal IGF2 gene showed an increase in birth weight. IGF2 expression was found to be lower in small for gestational age (SGA) babies.
What Does This Mean?
This research suggests that genes inherited from both parents regulate a baby’s growth, at different times during the pregnancy.
A widely accepted explanation for the evolution of gene imprinting is the ‘parental conflict hypothesis’.
This theory suggests expression of the paternal genes enhances fetal growth, improving the rate of success of the father’s genes being passed on to his offspring.
The expression of the maternal genes is to conserve resources, to ensure she is able to survive birth, as well as nourish her offspring, and reproduce again. Therefore, maternally expressed genes tend to be growth limiting.
The study from ICH suggests the genes from mother and father are acting at different times during pregnancy to control a baby’s size. The father’s genes seem to promote increased fetal growth, at an early stage during pregnancy, and the mother’s genes need to regulate this growth carefully, to ensure a successful birth.