Is my baby too small?
While large babies often grab the headlines, the other reason for monitoring a baby’s growth is to look out for babies who may be too small.
What determines baby’s size?
The two main factors influencing a baby’s size are genetics and environment. Each of us has a genetically pre-determined growth potential (given to us by our parents) which is influenced by the ethnicity and the physical characteristics of our parents (tall parents tending to have tall children and vice versa).The environmental factors that influence a baby’s size include general nutrition and the physical health of the mother.
What can cause a baby to be too small?
When thinking about small babies, there are two basic categories they fall in to. Babies that are “constitutionally small” are “small but happy” babies. They are achieving their genetically determined growth potential which just happens to be smaller than average.
Babies that are “growth restricted” are babies who are not achieving their growth potential, and there are a number of potential causes for this including chromosomal abnormality, structural anomalies, multiple pregnancy, infection, placental problems and maternal factors including some medical conditions or pregnancy complications (e.g. preeclampsia), smoking and substance abuse.
How can we estimate a baby’s size before it’s born?
An estimation can be made clinically by palpating a mother’s abdomen and measuring the size of the womb (using a tape measure to determine the symphysio-fundal height). This measurement, along with the rate of change over the weeks of pregnancy, can raise suspicion of a small baby if it is significantly lower than expected. Asking a woman who has had children before how the size of her current baby compares to previous pregnancies at the same stage can also be a useful guide.
An objective measure can be made by way of ultrasound examination where measurements are taken of particular parts of the baby (e.g. the circumference of the head and belly and length of the thigh bone). There are complicated formulas that use these measurements to work out what the “estimated fetal weight” is. These estimates are based on general population data and there are inherent errors in these measurements which is why we call the fetal weight “estimated”.
If a baby is suspected to be small for gestational age, then serial scans a couple of weeks apart are often performed to track the rate of baby’s growth to see if there is any reduced rate of growth. If it hasn’t been performed already, a detailed anatomy ultrasound of the baby should also be performed. Rarely, invasive tests like amniocentesis may be considered if infection or chromosomal problems are suspected.
What can be done if baby is growth restricted?
Reversing any reversible causes (smoking) and managing medical complications is the first step. It may reach a point where monitoring indicates that the baby would be better off outside the womb, and sometimes early delivery is indicated.
Early detection and management of growth restriction is important as there is evidence that babies who are are growth restricted are at a higher risk of developing health issues (e.g. high blood pressure) later in life.
The above information doesn’t take the place of a medical consultation so please seek further advice if your symptoms continue to concern you.