Small babies and big babies
The average newborn baby weighs in at 7lb 8oz – if yours is smaller or larger than this he’ll be classed as small or big for dates. As a rule, babies who weigh less than 5lb 8oz are considered to be low birthweight, while those who weigh less than 3lb 5oz are very low birthweight. Babies weighing more than 9lb 15oz are considered much larger than average. However the methods used to calculate your baby’s size before he is born can only estimate his likely birthweight, and several factors might render the calculations incorrect.
How your baby’s size is measured
In the first trimester (weeks 0-12) it’s thought that all babies grow at pretty much the same rate. During your first
ultrasound scan, which will be carried out during your first trimester screen around weeks 9-11, the sonographer will measure your baby’s length from crown to rump (head to bottom) as well as measuring the length of his limbs and the circumference of his head. These measurements are used to estimate how far along you are in your pregnancy. Your second ultrasound scan, carried out during your second trimester (weeks 13-28) also will measure your baby’s size, and the figures will be used to calculate how much he is likely to weigh when you reach your due date.
Your baby’s growth also is monitored at your routine antenatal checks, by your midwife. From around week 20 they will start measuring the distance between your pubic bone and the top of your uterus (called the fundus) and provide an assessment of how far along your pregnancy is – the fundal height, in centimetres, should roughly equate to how many weeks along you are in your pregnancy (at 20 weeks you’d be expected to measure 18-22cm). They also will palpate (feel) your bump to assess your baby’s size as well as his position.
Measuring small or large for dates
If your fundal height isn’t in the expected range according to your due date you’ll be assessed as either small (measuring 2cm or more less than expected) or large (measuring 2cm or more larger than expected).
It’s important not to panic – it’s almost certain your baby is perfectly healthy and growing as he should be. Keep in mind that your measurements are being compared to an average figure and since every pregnancy and baby is different, there are bound to be discrepancies. Your midwife will likely refer you for further investigation but this will likely be purely a precaution.
If your baby is large
There are several reasons why your baby might measure as large. The most likely explanation is that your due date isn’t correct and that you conceived your baby earlier than you first thought.
Alternately you could be measuring large because this is your second baby and your abdominal muscles are looser than they were first time out, so your uterus isn’t as tightly contained, making the fundal height longer. Baby boys also tend to be larger – and if you or your baby’s father were large at birth, your baby is likely to take after you. So it may be that you're having a big, but healthy, baby.
You also may measure larger if you have fibroids (harmless growths in the uterus), if you have an excessive amount of amniotic fluid, your baby is lying high in the uterus (this is more likely if you have
placenta previa or your baby is
breech) or you’re expecting twins or more. Obesity and/or gestational diabetes also can cause your baby to grow larger than average.
If your baby is small
Similar to factors that might mean your baby measures larger than expected, he may appear small because you aren’t as far along in your pregnancy as you thought. If it’s your first baby – and if you’re particularly fit – your abdominal muscles will be tighter, and will do a better job at holding everything in! If your baby is a girl, she will be smaller than a boy baby at the same stage of pregnancy, and if you were a small baby she may well just be following in your footsteps. So you could simply be carrying a small but healthy baby.
Other factors that might cause your baby to be small include too little amniotic fluid, and intrauterine growth restriction (IUGR). This can occur if you smoke in pregnancy, if you’re poorly nourished (for example, if you have severe
morning sickness) or if you have pre-eclampsia, which impedes the flow of oxygen and nutrients through the placenta to your unborn baby.
What it means for the birth
If you’re told your baby is large you likely will panic at the thought of delivering him vaginally but try not to. A true cephalo-pelvic disproportion (where a baby’s head is too big to fit through the mother’s pelvis) is very rare and most babes ‘fit’ their mums, no matter what their size.
Your baby’s position will be an important factor in how easily he is born – in fact, a large baby in a good position is easier to deliver than a small baby in an awkward position. His head has soft spots called fonatanels, where the skull bones have not yet fused together, and these overlap during his birth journey, ensuring that his head moulds to the shape of the birth canal. And pregnancy hormones relax and loosen the ligaments in your pelvis, enabling it to open up and allow him passage. Squatting during your labour and delivery also can help – it’s thought to open up your pelvis by as much as 30 percent.
That said, there is a higher risk of complications if your baby is large. If he is large because you have gestational diabetes, he may need to be induced earlier than his due date to ensure he doesn’t get too much bigger. You may experience a greater degree of perineal tearing or need an episiotomy or assisted delivery to get a large baby out. If your midwife suspects a particularly large baby they may advise you to have a
caesarean delivery, but be sure to discuss the risks involved with a c-section.
A baby who is small also may need to be delivered by caesarean if your midwife thinks the stress of labour might be too much for him. Low birthweight babies may have breathing difficulties after the delivery, and need to go to the
Special Care Birth Unit. Very low birthweight babies are at risk of severe problems that can cause feeding difficulties and problems in maintaining a warm enough body temperature.
The information in this feature is intended for educational purposes only. If you have any concerns about your health, the health of your child or the health of someone you know, please consult with a doctor or other healthcare professional.
Last Modified: 18/02/2009