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Sleeping and Growing
Pregnancy

Sleeping and Growing

2 min readWeek 34
Key takeaways
2 min
  • Understand that your baby's brain is nearly complete at 33-34 weeks, with developing sleep cycles including REM sleep crucial for reflex formation.
  • Recognize that soft spots (fontanelles) on your baby's head are normal and necessary for brain growth and safe delivery through the birth canal.
  • Know that babies born between 32 and 37 weeks are considered moderate to late preterm by WHO standards and may need special care after birth.
  • Observe on ultrasounds how your baby may be sucking their thumb, showing advanced motor skill development and self-soothing behaviors.

At 33-34 weeks, your baby's brain is nearly complete with developing sleep cycles including REM sleep. The skull remains soft with fontanelles to allow safe delivery, and babies born at this stage are considered moderate to late preterm by WHO standards.

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Sleeping and Growing

The baby’s brain is almost completely formed [1]. The pupils change size depending on light and the bones are getting stronger. The skull remains soft because the cranial plates have not yet fused [1], this will make it possible for the baby to pass through the narrow birth canal [1, 2, 3].

Sleeping and Growing - Sleeping and GrowingWhen your baby is born, you will notice that some parts of his head are very soft. These non-ossified areas of the cranial vault are called fontanelles [1]. They allow the skull to remain flexible as the brain grows [4].

As the central nervous system matures, the baby develops cycles of activity and rest. In the womb, they spend most of their time in REM sleep, which is characterized by rapid eye movement and is important for the formation of reflexes. At this stage in pregnancy, another type of sleep can be detected, during which the eyes move at a slower pace. Both phases of sleep are characterized by specific electrical activity in the brain, which can be monitored by an electroencephalogram (EEG).

If you are expecting twins

Even if labor starts today, try to stay calm, but call your doctor or maternity unit right away. By WHO standards, babies born between 32 and 37 weeks are considered moderate to late preterm, and twins are more likely than single babies to need special care after birth.

What can be seen on ultrasound

In the image, the baby is lying on their right side towards the screen of the ultrasound machine, giving us a close-up view of their profile. You can see the outlines of the forehead and chin. To the right, the baby’s arm is bent at the elbow and the palm and phalanges of the fingers are visible. At the moment the picture was taken, the baby was probably sucking on their thumb.

Sleeping and Growing - Sleeping and Growing
  1. head


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Frequently asked questions

Soft spots called fontanelles allow your baby's skull to remain flexible for brain growth and safe passage through the birth canal. These non-ossified areas are completely normal and will gradually close as your baby grows.

Babies spend most of their time in REM sleep at this stage, characterized by rapid eye movement. They also experience slower-paced sleep cycles, both crucial for brain development and reflex formation.

According to WHO standards, babies born between 32 and 37 weeks are considered moderate to late preterm. They are less vulnerable than babies born earlier but may still need special care after birth, so if you think labor is starting, call your doctor or maternity unit right away.

Medical Disclaimer

This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or qualified health provider with any questions you may have regarding a medical condition.

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Medically reviewed content

Reviewed by healthcare professionals · Updated

This article is based on peer-reviewed research and trusted medical sources.

  1. Brewer S. The Pregnant Body Book. Dorling Kindersley Publishing Staff, 2012, p. 163, 167, 180.
  2. Week-by-week guide to pregnancy. NHS.
  3. Fetal development: The 3nd trimester. Mayo Clinic.
  4. 33 weeks pregnant: fetal development. BabyCenter.
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