Understanding the Three Trimesters

Pregnancy is a beautiful and complex journey that brings both joy and challenges to expecting parents. It's a period of intense physical, emotional, and psychological transformation that lasts for about 40 weeks, divided into three trimesters. Each trimester is a unique phase of pregnancy that comes with its own set of changes and experiences. Understanding what happens during these trimesters can help expecting parents navigate the journey more smoothly and confidently. So let's dive into the three trimesters of pregnancy and learn what to expect during each stage.

First Trimester

Congratulations, new parents! You're about to embark on an exciting journey as your body begins to create and nurture a new life. In the first trimester of pregnancy, fetal development begins soon after conception. The estimated due date is calculated by counting 40 weeks from the start of the last period.

What Happens?

  • Fertilization. When sperm meets the egg a zygote with 46 chromosomes forms.
  • Implementation. From there, it travels down the fallopian tube, divides, creating a z cluster of cells called a blastocyst, and then attaches to the uterine lining.
  • Formation of the placenta. The inner group of cells will become the embryo, while the outer layer will form part of the placenta.
  • Formation of three embryonic layers. As the embryo begins to develop, it forms three layers: the ectoderm, mesoderm, and endoderm. The ectoderm will give rise to the nervous system, skin, hair, and nails. The mesoderm will form the bones, muscles, and reproductive system, while the endoderm will form the lungs, liver, pancreas, and digestive system.
  • Development of limbs, organs, and other structures. In the span of just a few weeks, your baby will begin to develop what will become their brain, spinal cord, head, limbs nose, ears, eyes, and digits.
  • Transition from embryo to fetus. After 8 weeks of gestation, the baby is no longer considered an embryo but a fetus. This signals that the baby is in a new stage of development that will continue until birth. Around this period, your baby will begin to develop external genitalia and soon after, fingernails. By the very end of the first trimester, you may even be able to hear your baby’s heartbeat.

Second Trimester

The second trimester generally marks a turning point in pregnancy, with the fetus developing all its organs and bodily systems and beginning to grow in length and weight. Luckily, the second trimester is often said to be the most physically enjoyable for the birthing parent. Moreover, the second trimester is full of exciting developments that will allow you to bond with your baby even while they are in utero!

What Happens?

  • Sex Becomes Apparent. After the 14th week of pregnancy, the baby’s sex may be able to be visualized in an ultrasound. But be patient, it is perfectly normal for this to take closer to 20 weeks.
  • Baby’s Movement Can Be Felt. Beginning as early as 16 weeks, you may begin to feel your baby moving! While at first, this may be a bit uncomfortable, feeling your baby move is often cited as one of the most exciting and endearing milestones in pregnancy. At this point, your baby is most likely moving to exercise their recently formed limbs but as pregnancy progresses you may also notice them move or kick in response to stimuli.
  • Hearing Develops. Around 18 weeks, your baby’s ears begin to shift into place and stick out at the sides of the head. What’s more, they will start to perceive sounds! Babies can best hear the voice of the parent carrying them but they can also hear other voices and noises around them. This is a great time to start bonding with your little one by talking and singing to them!
  • Reflexes Develop. Soon after hearing, other crucial abilities like sucking and swallowing develop. You may even see your baby sucking their thumb in an ultrasound!
  • Viability. At the end of the 2nd trimester (after 24 weeks), most babies are considered physically developed enough that they could survive outside of the uterus, with extensive medical intervention. Reaching this milestone may be a relief to parents, especially those who have experienced health difficulties during pregnancy or are expecting a premature baby.

Third Trimester

As you enter the final stretch of pregnancy, fetal development continues at an astonishing pace. You’re in the home stretch! Throughout the third trimester, it's essential to take good care of yourself especially as you notice physical tasks becoming more challenging. Get lots of rest, water, and nutrients, and don’t be afraid to ask for help. Do your best to relax and get ready to welcome your little one into the world!

What Happens?

  • Central Nervous System Development. In the 3rd trimester, your baby’s body is becoming more and more self-supporting. At 28 weeks, the central nervous system can control rhythmic breathing movements and body temperature.
  • Changes in Movement. Around week 29 you may notice more kicking and turning which will continue for several weeks and taper off around week 36 as your baby takes up more and more space in the amniotic sac.
  • Vision Starts to Develop. Around 30 weeks, your baby's eyes can fully open, and soon after their pupils will be able to respond to stimuli caused by light!
  • Baby’s Hair Grows. At the same time that vision is developing, your baby may be well on their way to growing a full head of hair! Every baby is different and some may be born with almost no hair at all, but at this point in pregnancy, you may be able to visualize hair in an ultrasound.
  • Position Changes. At 37 weeks, your baby might turn their head down, preparing for their big debut into the world! If your baby is still feet-first as you approach your due date, talk to your healthcare provider about your birthing options.
  • Due Date Arrives. At 40 weeks you will come up on your due date! Many parents may already have welcomed their baby before their due date arrives and many may see their due date come and go with no activity. Both are perfectly normal! Just make sure you’re communicating with your doctor and monitoring yourself for symptoms of labor. You got this!


“Pregnancy Week By Week.” Mayo Clinic, 3 June 2022, www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302.

Breborowicz GH. Limits of fetal viability and its enhancement. Early Pregnancy (Cherry Hill). 2001 Jan;5(1):49-50. PMID: 11753511.

“The Second Trimester.” Johns Hopkins Medicine, 8 Aug. 2021, www.hopkinsmedicine.org/health/wellness-and-prevention/the-second-trimester.

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