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Am I Having a Big Baby?

Is my baby a big baby?

A commonly asked question (especially getting closer to the day of delivery) is “How big is my baby?”. It’s not surprising that the issue of baby’s size can weigh on Mum’s mind (pardon the pun) as often expectant mothers have heard many stories of “10 pounders” being delivered and all that the process involved!



What determines a 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. Some pregnancy complications including maternal diabetes, increased weight gain during pregnancy and pre-existing obesity, post-term pregnancy, and some rare genetic syndromes can lead to mothers having larger babies.

Male babies and subsequent children also tend to be larger.


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 large baby if it is significantly higher 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 measurements which is why we call the fetal weight “estimated”.

When is a baby a “big” baby?

In one sense, that’s a bit like asking how long is a piece of string as a baby weighing 3.5kg may be normal for one mother, but large for another (due to the factors I mentioned above).

In general, however, any baby that is measured by ultrasound to have an estimated weight of more than the 90th centile for its gestational age is determined to be large for gestational age (LGA) and any baby that is born with a weight of 4.5kg or more is defined as macrosomic.

Are there any risks with carrying and delivering a large baby?

There are some risks to mothers (long or obstructed labour, vacuum or forceps delivery, caesarean section, genital tract trauma, post-partum haemorrhage) and babies (shoulder dystocia, low blood sugar after delivery).

The decisions around timing and mode of delivery are best made on a case by case basis after careful consideration of all the potential risks and benefits of the different options available.

Is there any way to avoid LGA or macrosomia?

We can do nothing about our genetics, but modifying lifestyle factors and managing diabetes as well as possible can reduce the risks of babies gaining excess weight.

The above information doesn’t take the place of a medical consultation so please seek further advice if you have specific questions or concerns.

Download a PDF copy of this article here

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