Small or large for dates - what does that mean?

We explain what it means when your doctor describes your baby as small for dates or large for dates.

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Pregnancy health > Small or large for dates - what does that mean?

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As part of your antenatal care your doctor or midwife will regularly monitor your baby's size to check that they're developing and growing healthily. This is usually done by measuring your fundal height, the distance in centimeters between the top of your pubic bone and the top of your uterus, and gives your health care provider a rough estimation of both the size of your uterus and the baby it nurtures.

Rather amazingly, for most of your pregnancy your fundal height will be roughly equal to the number of weeks pregnant you are, usually with an accuracy of 2cms. So, if you're 15 weeks pregnant your fundal height is likely to be somewhere between 13 and 17cm. It is because of this similarity, that fundal height provides a relatively reliable, non-invasive measure of development for healthcare providers to use.

When measuring your fundal height it is possible that your midwife will tell you than you are measuring 'small' or 'large' for your dates. This can be alarming however to stop you worrying unnecessarily we explain what exactly measuring small or large for your dates actually means.

Measuring small for your dates

What does it mean?

Measuring small for your dates means that your fundal height is reading smaller than would be expected compared to that of the 'average' pregnancy.

Why might that be?

There are a whole host of reasons why you might be measuring small for dates and your midwife is going to want to investigate further to check that everything is ok.

The main concern will be that your baby is developing as he or she should. There is a small risk that they could be suffering from an Intrauterine Growth Restriction (IUGR), potentially indicating a problem with the placenta such as pre-eclampsia which would require treatment. However, as your fundal height will be measured against averages and, as every Mum-to-be is shaped differently and every developing baby grows at a different rate, it could simply be that your baby is due an in-utero growth spurt or that you have particularly strong abdominal muscles that means you to carry your baby differently to most women.

What happens next?

To make sure that your baby is growing as he or she should your midwife is likely to send you for further investigation just to check that all is well. An ultrasound is likely to be the next step as it will be able to accurately date your pregnancy and identify any potential causes to explain why your measurements are coming up small. If, at the first ultra scan your baby is measuring slightly small, it's likely that you'll be asked back for a rescan a couple of weeks later to monitor your baby's growth during that time. Your midwife may also suggest a doppler ultrasound as this will measure your baby's heartbeat and check that the placenta is working as it should.

Measuring large for dates

What does it mean?

Measuring large for your dates means that your fundal height reads more than 2cm more than would be expected for your stage of pregnancy compared to the average.

Why might that be?

Again, there are a number of reasons why you might be measuring larger than expected and your midwife will want to carry out a few more checks to ensure that everything is progressing as it should be. For instance your abdominal and uterine muscles may be more relaxed so that your bump shows more, this is particularly likely if you have carried other children. If you're earlier on in your pregnancy it may be that you are carrying twins. You may on the other hand simply be carrying a very large, healthy baby. It is also possible that your due date has been miscalculated and you're further on in your pregnancy that previously thought. However, occasionally measuring large can be an indication of uterine fibroids or gestational diabetes so it's likely that your midwife is going to want to rule these out.

What happens next?

This really depends where you are in your pregnancy. It's more common for women to be large for their dates as they near the end of their pregnancy as subtle differences in size become more defined. However, whatever your stage you may be sent for an investigatory ultrasound to confirm your due date, check whether you'll be Mum to one or two and generally make sure that everything is ok.

Should I be worried?

Whether you're measuring large or small you should try not to worry yourself unnecessarily. In all likelihood your baby is growing and developing just as they should be and the discrepancy is simply down to the individual differences that sometimes occur when you compare a person to an average. If your doctor or midwife refer you for further investigation it's only to make sure that everything is ok and shouldn't be taken as a sign that something is definitely wrong. Staying calm, looking after yourself and enjoying your pregnancy is the best thing that you can do for yourself and for your baby.

Whether you have any questions about your pregnancy or just fancy a chat, why not visit the AskBaby forums for a chat?