Why Do They Call It The Fourth Trimester?

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Fourth Trimester

The fourth trimester describes the period from when your baby is born until three months after their birth which you may find the term a bit strange as your baby is already born. However, human babies enter the world completely dependent on caregivers to provide for their every need. Compared with other primates, the brains of human babies are relatively underdeveloped – in fact, for a baby to be born at the same neurological stage as a chimpanzee, the gestation period would need to be between 18 and 21 months!1 Um, no thanks – nine months is quite enough thanks. So why are humans born so underdeveloped and so helpless.

The most widely accepted scientific explanation is that due to our ancestors starting to walk upright, resulting in restrictions to the width of the birth canal, as well as our large brain size, natural selection favored childbirth at an earlier stage of development. In fact, human babies are born when their brains are less than 30 percent the size of an adult’s brain, in order to be able to fit through the narrow birth canal. However, other recent explanations include a limit on how long the baby can be sustained in the womb, as this takes a huge amount of energy away from the mom.1

Why is the fourth trimester important for the baby?

The fourth trimester is a time for your baby to get used to life outside the womb. Although your baby’s brain is well developed at birth, his or her neural pathways and nervous system continue to develop, with most of the changes occurring in the fourth trimester. In the first three months of life, you may notice that your baby is:

  • Settling into more consistent sleep and feeding patterns
  • Developing more controlled movements and startling less
  • Breathing more steadily
  • More able to sleep through noise and disturbances
  • Learning to soothe herself or himself
  • Improving their social skills so they can interact with people with greater attention and for longer periods of time2

What differentiates the fourth trimester to later periods of development?

Your baby is likely to cry more during the fourth trimester, compared with other development periods. Crying tends to peak at around five to six weeks and usually subsides by the time your baby is around three months old. In addition, your baby will be sleeping a lot during this time, particularly in the first few weeks. Sleep is super important for your baby at this stage as it helps his or her brain to process all that sensory input that he or she is being exposed to while they’re awake. During this period, your baby will not be likely to have a sleeping pattern and for the first few weeks or months, they will not be able to tell night from day.

Feeding is another area in which the fourth trimester differs from later periods of development. As a newborn, your baby has a small stomach and consequently he or she needs at least eight feeds every 24 hours. Signs that your baby is hungry include:

  • Sucking on fingers
  • Turning his or her head
  • Opening his or her mouth
  • Crying2

Crying can often be the last sign that your baby is hungry so try to watch for the earlier signs first – if your baby reaches the point of crying, he or she may be too upset to latch on to your breast properly or to settle down for a feed.

During this period, your baby needs to be fed on demand as they are too young for a feeding routine just yet. Don’t worry about spoiling your baby – on the contrary, it will help he or she feel more secure and maybe even cry a little less.

Although your baby’s senses are quite well-developed at birth, they will continue to improve throughout the fourth trimester. In particular, sight develops most rapidly during this time and by eight or nine weeks, your baby’s vision will be almost as good as an adult’s vision.

By the time your baby is born, he or she will be able to recognize the sound of your voice and maybe that of your partner’s too. As a result, your baby will enjoy listening to you talking and will feel soothed by familiar noises and sounds.2

Methods to help your baby through the fourth trimester

There are certain things you can do to help your baby make the transition from inside the womb to the outside world. These include:

  • Swaddling – this mimics the womb in that your baby feels contained, and may help your baby sleep better and soothe him or her if they are crying
  • Skin-to-skin – your baby will be reassured by your smell and warmth, as well as the familiar sound of your heartbeat; skin-to-skin also helps your baby latch on for breastfeeding
  • Feeding on demand – in the womb your baby was constantly supplied with nourishment from the umbilical cord and by feeding on demand you are mimicking this constant supply of food
  • Baby wearing – wearing your baby in a sling recreates the movement your baby would have felt in the womb and may comfort him or her if they are crying
  • A warm bath can help soothe your baby and mimic the womb2,3

The fourth trimester is challenging, for babies and parents alike, but it is also a critical period of development and like so many other periods in your child’s life, it really is just a phase, albeit a very important one. Being aware of what your baby is going through and helping your baby transition during this time might make this period a bit easier to cope with.

References:

  1. Why humans give birth to helpless babies
  2. Your baby and the fourth trimester
  3. The fourth trimester
Melody Watson
Melody Watson holds Bachelors degrees in Biochemistry and Microbiology. She works as a medical writer for a medical communications agency in Berlin, Germany, where her work ranges from medical translation to writing publications for medical journals. Melody is passionate about promoting science, including evidence-based medicine, and debunking pseudoscience.

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