Big or small for dates

During the course of your pregnancy you may well be told by your OB-GYN or nurse-midwife that you’re ‘big’ or ‘small’ for dates. So what does this mean, and should you be worried?

As your pregnancy progresses, your caregiver will assess your baby’s growth by measuring the distance from your pubic bone to the top of your uterus – it’s known as ‘fundal height’. It’s measured in centimeters and should roughly equate with how many weeks pregnant you are – so, if you’re 23 weeks pregnant your fundal height might be 21-25cm.

A more accurate assessment can be done by ultrasound scan, and you may be given one of these if you appear to be significantly over or under where you should be in terms of size.

Why might I be small for dates?

Small for dates is defined as being more than 2cm below the average fundal height for your stage of pregnancy. There are many reasons why you might measure as small: it may simply be because you’re not as far along as you thought. Other reasons can include strong abs or being small yourself – in which case your baby might just be naturally small too.

But measuring small could also indicate a more serious problem, such as too little amniotic fluid or intrauterine growth restriction because your baby isn’t getting the oxygen and nutrients he needs to grow. This could be due to a deficient or low-lying placenta (placenta previa), high blood pressure, pre-eclampsia, a poor diet or even certain medications. Chromosomal abnormalities such as Down syndrome can also affect your baby’s size.

If your OB-GYN or nurse-midwife is concerned, you’ll be referred for further tests to determine the cause.

Why might I be big for dates?

Big for dates is when your fundal height is more than 2 centimeters over the average for your stage of pregnancy. Again, this could be due to a number of factors. Many women on their second and subsequent pregnancies appear large for dates because their abdominal muscles are often looser than those of first timers. It may also be because you’re carrying a lot of amniotic fluid or because your baby is simply sitting high above your pelvis. A high position can sometimes indicate problem such as a breech presentation, and your midwife will want to investigate this further.

Other causes could be a multiple pregnancy, or a condition known as gestational diabetes, which can be diagnosed with a simple test. And again, it may simply be that your due date has been wrongly calculated, and you’re more pregnant than you think. Of course, you may just also be carrying a very big and healthy baby!

Have you tried out our due date calculator to check when you’re baby might be born? Simply type in the first day of your last period and how long your cycle usually lasts and our calculator will give you an estimated due date.

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.