Too Big Or Small For Gestational Dates?

Too Big Or Small For Gestational Dates?

Being told your baby is small or too big for gestational dates can be confusing and upsetting. But in most cases there is no reason to be alarmed.

If you look around any antenatal class you will see women at the same stage of pregnancy sporting bellies in a range of shapes and sizes.

Too Big Or Small For Gestational Dates?

More often than not, the size of your baby is related to your own shape, and to the genes your baby is favouring (Is Dad tall? Is Mum petite?)

Big or small, does it really matter?

Sometimes there are underlying causes as to why your baby is big or small for dates, but other times it is simply normal for your baby.

How Is Baby’s Growth Measured?

Measuring unborn babies is actually quite difficult. In the past, maternal weight was used as a guide to determine the size of the baby, but this is no longer considered reliable; a pregnant woman can still gain weight even if her baby is not growing.

The most common method is to take a fundal height measurement. This is the distance between your pubic bone and the top of your uterus. Your care provider will use a measuring tape and place one end on your pubic bone and stretch it up over your growing belly to where the top of your uterus has stretched out of the pelvis.

After 16 weeks gestation, your fundal height measurements should roughly match the number of weeks you have been pregnant. So at 20 weeks, your fundal height measurement will be about 20 centimetres.

At about 20 weeks gestation, your care provider will begin to measure your baby’s growth at regular intervals. As your pregnancy progresses and your baby grows, it is expected your fundal height will also increase.

From about 20 weeks, your baby’s growth and size become more individual. Not all babies grow steadily; some have jumps and spurts, and others might be slow to start and pick up later on in pregnancy. A fundal height measurement is a ‘rule of thumb’, meaning it is simply a guide for your care provider to make sure your baby is growing.

Depending on the situation, your care provider might decide to take measurements again two weeks later, or refer you to have an ultrasound and other tests, to determine what is causing the different measurements.

What Does ‘Large For Gestational Age’ Mean?

If your fundal height is more than 3 centimetres higher than expected for your gestational dates, your care provider might decide your baby is ‘large for gestational age’.

It is likely you will be recommended to have an ultrasound, to check for any reasons why your baby is ‘too big for dates’.  There might be an underlying cause for your baby’s growth, or it might simply be normal for your baby to be this size at this gestation.

Reasons why you could be measuring large for gestational age:

  • Your estimated due date is incorrect
  • You have had previous pregnancies and your abdominal muscles are stretched
  • You have type 1 or 2 diabetes, pre-pregnancy, or have developed gestational diabetes, all of which increase the chances of a bigger baby
  • You have uterine fibroids which have grown and are taking up some uterus room
  • You are pregnant with multiples
  • You have a high body mass index, which can make it more difficult to get an accurate measurement
  • There is too much amniotic fluid (polyhydramnios)
  • The baby has adopted a breech, or other unusual position
  • Your body shape

What Does ‘Small For Gestational Age’ Mean?

Your care provider might decide your baby is ‘small for gestational age’ if your fundal height measurement is more than 3 centimetres lower than expected for your gestation.

For a baby measuring smaller than gestational age, it is likely your care provider will recommend an ultrasound, to determine whether there is a underlying problem, or whether your baby is simply petite. Some babies save their growth spurts for later on in pregnancy.

Reasons you could be measuring small for gestational age:

  • Your estimated due date is incorrect, due to irregular cycles, or not knowing the date of your last menstrual period
  • Intrauterine restricted growth
  • Too little amniotic fluid (oligohydramnios)
  • You have very strong abdominal muscles
  • Your body shape
  • Your baby has already adopted a head down and engaged position.

If your baby doesn’t seem to be growing well, you will be given a follow up ultrasound after 2-3 weeks. This will check for growth and amniotic fluid levels. You might also need additional monitoring as your pregnancy progresses.

How Does Fundal Height Measurement Affect My Birth Experience?

In many cases, your baby is growing perfectly well and there should be no reason you can’t go on to have a normal labour and birth.

It’s important to note ultrasound accuracy for weight is highly variable, particularly in the third trimester, and especially when it comes to bigger babies. Your care provider might recommend regular ultrasounds to check your baby’s growth, but it is not an exact science and there is always room for error.

If your baby is measuring large for gestational dates, you are likely to be faced with pressure to induce or possibly have an elective c-section at early term (37 weeks). There is a lot of fear around giving birth to a large baby, and women are often told there are too many risks to allow their pregnancy to go to full term.

You can read more about big babies in Macrosomia – 5 Myths About Big Babies and Birth and What Is Considered To Be A Big Baby?.

Babies measuring small for dates might need more monitoring during labour, as smaller babies can struggle to cope with contractions. If your baby’s growth is restricted because of a health condition, and gestational age is greater than 34 weeks, your care provider might recommend being induced. If you are less than 34 weeks pregnant, you might need extra monitoring to keep an eye on your baby’s wellbeing.

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Sam McCulloch Dip CBEd CONTRIBUTOR

Sam McCulloch enjoys talking so much about birth that she decided to become a birth educator and doula, supporting parents in making informed choices about their birth experience. In her spare time she watches Downton Abbey and has numerous creative projects on the go. She is mother to three beautiful little humans.


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