1 Month
Two important events occur, including forming the amniotic sac and the placenta. While the first protects the embryo from any possible physical damage or drastic temperature changes, the placenta serves as a channel for the nutrition and wastes coming in and out of the sac.
7 Weeks
The cells in the neural tube, including the brain and spinal cord, develop as the head becomes larger than the rest of the body. Bones slowly begin to replace cartilage and limps grow out more. The immune system starts to develop as early as week four and continues to evolve, according to Park et al. (2020), as “The B cell lineage is first observed in the fetal liver from 7 PCW in the form of B cell precursors, and mature B cells are present only after 9 PCW” (p. 602).
3 Months
From this month, the embryo is commonly referred to as a fetus, and by the end, the fetus can close its mouth and fists. Fingernails, toenails, teeth, and even the reproductive system are beginning to form and develop, while urinary and blood circulatory systems are already functional. By the end of this month, all the limbs and organs are not yet fully developed but already present. The immune system’s T-cells form in the thymus and spread to other organs, “Circulating T cells are observed at 10-11 PCW following functional thymic development” (Park et al., 2020, p. 603).
4 Months
This month is when the heartbeat becomes audible through a Doppler, and the biological sex of the fetus can be determined using ultrasonography. The nervous system starts functioning for the first time. The facial muscles are developed enough that the fetus can change its expression.
5 Months
During this month, the body muscles develop, and the fetus starts to move around the sac. Hair grows on the fetus’s head. Additional hair, lanugo, growing on the back, temples, and shoulders, and white skin coating, vernix caseosa, further protect the fetus’s skin.
6 Months
The fetus can now react to sounds by moving, or its pulse increases. The finger and toe prints are fully visible. The taste buds, eyelashes, and eyebrows are developing. By the end of this period, the prematurely born baby can survive intensive care.
7 Months
Now the fetus’s hearing is fully developed, and it also starts to gain body fat reserves. The fetus senses the environmental elements, including sound, light, and pain, and responds by changing its position. The immune system antibodies start to pass through the placenta to the fetus from its mother. The amniotic fluids decrease in volume as the body slowly prepares for labor. After this month, the baby is likely to survive premature birth.
8 Months
Most of the organs and systems are fully developed, except the lungs, in some cases. Now the fetus can see and hear fully. The body fat reserves continue to grow. The fetus becomes more active, and pregnant women can experience more movement and kick.
9 Months/Newborn
The lungs are finally nearing full development. The brain matured enough that the fetus shows the signs of coordinated reflexes, including turning its head, blinking, and closing its eyes. By this month’s end, women can enter labor at any time, as the fetus’s position is likely to change to prepare for it, with the head down the uterus being an ideal arrangement.
Reference
Park, J. E., Jardine, L., Gottgens, B., Teichmann, S. A., & Haniffa, M. (2020). Prenatal development of human immunity. Science, 368(6491), 600-603.