Part 2 - Macro/physical changes (from Weeks 3 - Birth)

By Angus Nicol

First Trimester
 
At a more macro, less cellular level, there are some significant milestones between weeks 3-8.
 
Weeks 4-5 - The heart usually starts beating between 4th-5th weeks and all the muscles and blood vessels in our body branch outwards from our growing, beating heart. Eye pits form, limb buds become apparent, first signs of pulmonary system are formed (connects heart to lungs).

Week 6 - Uncontrolled foetal limb movements start to occur, paddle-shaped fingers/toes undergo “apoptosis” (programmed cell death) so webbing disintegrates leaving defined fingers/toes.

Week 7 - Facial structure more complex. Includes nostrils, outer ears and lenses. By the end of Week 7, the foetus is distinguishable as a tiny alien-human.

Week 8 - Head as large as rest of embryos body, external genitalia become apparent (can’t distinguish M/F), cartilage begins ossification process into bone.

Week 10 - Organs like kidneys, intestines, brain, and liver are beginning to function, and fingernails and toenails are starting to form.

Week 11 - Red blood cells (carry oxygen vital for energy/life) start forming in liver.  By end of week, external genitalia apparent (penis or labia & clitoris). Bud for teeth appear. Now no longer classified as embryo, now classified as foetus.
​
Week 12 - Face has a more developed profile.

---

Second trimester
 
Week 13-16 - Sensory organ development. The eyes move closer together; blinking motions begin, although eyes sealed shut. Lips exhibit sucking motions. Scalp begins to grow hair. The excretory system developing: the kidneys well-formed; meconium (fetal feces) begins to accumulate in the intestines.
 
Weeks 16-20 - Fetus grows & limb movements become more powerful. Mother may begin to feel “quickening” (foetal movements - space restrictions force foetus into foetal position). Skin coated with waxy substance “vernix caseosa) for protection, moisturisation and lubrication during birth. 

Third trimester
 
Weeks 21–30 -  Rapid weight gain (important for maintaining stable body temp after birth). Bone marrow completely takes red blood cell synthesis, nerves undergo myelination (production of fatty protective sheath that helps electrical conduction - this process continues until adolescence). Fetus grows eyelashes. Eyelids not fused (can open/close). Lungs produce surfactant (substance that reduces tension in lungs, assists proper lung expansion after birth). Males: Testes descend into scrotum(towards the end). Fetus at 30 weeks measures 28 cm from crown to rump, exhibits approx body proportions newborn (but much leaner).
Baby may begin dreaming (28-32wks).

Weeks 31 - Birth -  Fetus lays down fat (fills out layer below skin aka “hypodermis”, skin transitions from red/wrinkled to soft/pink. Nails grow to tips of fingers & toes. At birth, average crown-to-rump length is 35.5–40.5cm, weight 2.5–4kg.