Once a pregnancy has been established and the embryo firmly ensconced in the uterus, the next phase of development begins. This is now the Fetal Stage and consists of growth and maturation of the structures that were formed during the embryonic period. This starts at 12 pregnancy weeks and ends after delivery at 37 to 42 pregnancy weeks.
12 pregnancy weeks
The uterus is just palpable above the pubic bone. The fetus is about 6-7 cm long. Centres of ossification or areas of bone formation have appeared in the fetus by the 12 pregnancy weeks. The fetus now has fingers, toes, skin and nails. The external genitalia have begun to show signs of being a male or a female. The fetus at 12 pregnancy weeks makes spontaneous movements but these are not yet felt by the mother. That usually happens after 18 pregnancy weeks.
16 pregnancy weeks
The fetus is now 12 cm long and usually weigh 110 g. The fetal sex can already be determined on ultrasound by experienced observers as they inspect the genitalia at 14 pregnancy weeks.
20 pregnancy weeks
At the end of 20 pregnancy weeks, the uterus is now at the level of the umbilicus or bellybutton. The fetus now weighs more than 300 g, and the weight begins to increase over time rapidly in a linear manner. The fetal skin is less transparent and already covered by lanugo (downy hair). The mother can identify fetal movements at 20 pregnancy weeks, described as fluttery sensations in her lower abdomen.
24 pregnancy weeks
The fetus now weighs about 630 g. The skin is wrinkled but fat deposition has begun, under the skin and elsewhere. The eyebrows and eyelashes can be recognized. The lungs have sufficiently developed such that a fetus born at this time will attempt to breathe. However, the terminal sacs of the lungs where oxygen is taken by the body have not yet formed and, thus the fetus will still die if born at 24 pregnancy weeks.
28 pregnancy weeks
The fetus now measures 25 cm long and weighs 1100 g. At this time the membrane over the pupils or the black of the eye has disappeared. The skin is thin, red and wrinkled. Babies born at this particular pregnancy weeks moves limbs and cries weakly.
32 pregnancy weeks
The fetus at 32 pregnancy weeks is 28cm and weighs 1800 g. The skin is still red and wrinkly. The lungs have matured enough that babies born at this time have a 90% chance of survival.
36 pregnancy weeks
The fetus is now 32 cm long and weighs 2500 g. By this time enough deposition of fat under the skin has occurred and the body is more rotund. Babies born at 36 pregnancy weeks have excellent chances of survival.
40 pregnancy weeks
The fetus is fully developed and has started “breathing”. “Breathing” in amniotic fluid or the fluid surrounding the fetus in the mother’s womb helps in further maturation of the lung itself. The fetus is now 28 cm long and the weight is 1800 g. The skin is still red and wrinkled. Babies born at this time have maximum chances of survival.
By knowing what is happening to your baby at different pregnancy weeks will help you visualize her and keep the mother-and-child bonding. Also, knowing what to expect next is a big help in the well-being of the mother. The mother can feel closer to her baby because she has an idea of what her baby is going through inside the womb.