The womb is a dynamic structure, with the ability to stretch to more than 5x its own size during the later stages of pregnancy(1). Prior to conception, the average womb size can be likened to that of a pear. However, throughout pregnancy, the womb gradually stretches and grows in size in order to accommodate the growing fetus.
This occurs due to the many physiological changes that happen during pregnancy, including weight gain, hormonal changes and increased blood volume(1). Anatomically, the female body is perfectly curated to accommodate the growth of a fetus, right the way through to 40+ weeks of pregnancy.
GROWTH OF THE WOMB
During your pregnancy, your midwife/other healthcare professionals involved in your care will monitor the growth of your womb and baby at regular intervals. This usually would occur during your dating scan (between 8-14 weeks gestation), 20-week scan (between 18-20 weeks gestation) and then again in roughly 3-4 week intervals depending on the nature of your pregnancy(2).
As well as measuring the womb and baby via ultrasound, healthcare professionals are able to estimate the approximate size of your womb externally by measuring what’s known as the ‘symphysis-fundal height’.
This is done by taking a tape measure and measuring from the upper margin of your central pelvic bone (pubic symphysis) to the highest point of your womb, known as the fundus(3).
Roughly from 20 weeks of pregnancy, the top of your womb is expected to reach your belly button. From 24 weeks of pregnancy onwards, it is expected that normal growth would find your symphysis-fundal height in centimetres would match the number of weeks pregnant you are, ± 2cm(4). Accuracy of these measurements may vary if for example you have a high BMI, more than one baby on board, or if you have a history of fibroids.
Fig. 1. Typical fundal heights at various stages of pregnancy.(3)
These measurements will usually be plotted onto a chart which should be personalised for your own ethnicity, height, weight, and weights of previous babies. This can help identify if your baby is growing bigger or smaller than expected. If there is any concern, an ultrasound will be arranged to take a closer look at the baby and the measurements.
CHANGES IN AMNIOTIC FLUID
During this period of growth, the baby isn’t the only thing getting bigger! There will also be growth of the placenta, as well as an increase in the amount of amniotic fluid produced. The amniotic fluid is there to cushion your baby, allow your baby to move around, prevent any pressure on the umbilical cord, in addition to many other purposes.
Generally, the volume of amniotic fluid produced in your womb gradually increases throughout pregnancy. The volume of fluid is approximately 10-20ml at 10 weeks, 250ml at 16 weeks, 800ml at 33 weeks, before reaching a peak of approximately 1000ml around 38 weeks of pregnancy. After this point, the volume of amniotic fluid begins to decrease back to roughly 800ml(5).
So what exactly is happening? Does this mean that the baby will begin to run out of space?
FETAL MOVEMENTS
Most people tend to feel their baby move between 16-24 weeks of pregnancy, with the first fluttering sensations being referred to as ‘quickening’. As the baby grows in size, movements and kicks become more pronounced, with a general increase in the number of movements up to roughly 32 weeks of pregnancy(6).
From this point on, it is possible that as the baby nears its largest size, the type of movements felt may differ from their norm. However, this is not to say that there will be a marked decrease in movements felt – you should continue to feel your baby’s movements up to and during your labour.
There isn’t a specific number of movements that should be felt as this varies from baby to baby. In addition to this, the time of the day can also affect how much your baby is moving, as they may be sleeping (yes, they do nap even when they are still in the womb! Their sleep usually lasts less than 90 minutes)(6).
So with this knowledge, what should you do if you feel your baby’s movements are noticeably reduced?
Less than 24 weeks pregnant? If you have never felt your baby move by 24 weeks, contact your midwife/maternity unit so that they can listen for your baby’s heartbeat and/or arrange an ultrasound scan.
Between 24-28 weeks pregnant? Ensure to do the same as above, where you can expect a full check up including blood pressure and urine tests.
Over 28 weeks pregnant? Contact your midwife/maternity unit immediately, without delay. This would allow for your baby’s heart rate to be monitored for 20 minutes or more, giving you some reassurance about their wellbeing. At this point the healthcare team will either send you home, or discuss any next possible options with you.
If at any point after you return home you notice a reduction in your baby’s movements again, do not hesitate to contact the maternity team again. You can come back as often as you need to!
SO, DO BABIES RUN OUT OF SPACE IN THE WOMB?
No – this is very much an old wives tale!
Whilst the in utero environment does become more snug during the third trimester compared to the earlier trimesters, the stretchy characteristics of the womb coupled with an adequate amniotic fluid volume allows for babies to still have space to move around, albeit not as pronounced.
If your baby’s movements are reduced, don’t assume it’s because the baby hasn’t got enough room!
Every baby has its own ‘normal’ regarding frequencies of movement. However, if you ever feel as though your baby is moving significantly less than what’s normal for them, do not hesitate to contact your midwife or maternity unit immediately.
REFERENCES
3. Collins S, Arulkumaran S, Hayes K, Jackson S, Impey L. Normal pregnancy. Oxford Handbook of Obstetrics and Gynaecology. 2013;:1-48.