Weeks 9 to 16 (2 to 3.6 months) , at three months gestational age In humans, the fetal stage starts nine weeks after
fertilization. At this time the fetus is typically about in length from
crown to rump, and weighs about 8 grams. Breathing-like movements of the fetus are necessary for the stimulation of
lung development, rather than for obtaining oxygen. The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation. Uncontrolled movements and twitches occur as
muscles, the
brain, and pathways begin to develop.
Weeks 17 to 25 (3.6 to 6.6 months) A woman pregnant for the first time (
nulliparous) typically feels
fetal movements at about 21 weeks, whereas a woman who has given birth before will typically feel movements by 20 weeks. By the end of the fifth month, the fetus is about long.
Weeks 26 to 38 (6.6 to 8.6 months) The amount of body fat rapidly increases. Lungs are not fully mature. Neural connections between the
sensory cortex and
thalamus develop as early as 24 weeks of gestational age, but the first evidence of their function does not occur until around 30 weeks. Bones are fully developed but are still soft and pliable.
Iron,
calcium, and
phosphorus become more abundant. Fingernails reach the end of the fingertips. The
lanugo, or fine hair, begins to disappear until it is gone except on the upper arms and shoulders. Small breast buds are present in both sexes. Head hair becomes coarse and thicker. The survival rate rises to over 90% for babies born at 25–27 weeks
weeks of gestation ( – months). The fetus is considered full-term between weeks 37 and 40. Birth is imminent and occurs around the 38th week after fertilization, followed by
life outside the uterus. It may be in length when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until
puberty.
Variation in growth There is much variation in the growth of the human fetus. When the fetal size is less than expected, the condition is known as
intrauterine growth restriction also called fetal growth restriction; factors affecting fetal growth can be
maternal,
placental, or
fetal. • Maternal factors include maternal
weight,
body mass index, nutritional state,
emotional stress,
toxin exposure (including
tobacco,
alcohol,
heroin, and other drugs which can also harm the fetus in other ways), and
uterine blood flow. • Placental factors include size, microstructure (densities and architecture),
umbilical blood flow, transporters and binding proteins, nutrient utilization, and nutrient production. • Fetal factors include the fetal genome, nutrient production, and
hormone output. Also, female fetuses tend to weigh less than males, at full term. SGA can result in
low birth weight, although premature birth can also result in low birth weight. Low birth weight increases the risk for perinatal mortality (
death shortly after birth),
asphyxia,
hypothermia,
polycythemia,
hypocalcemia,
immune dysfunction,
neurologic abnormalities, and other long-term health problems. SGA may be associated with growth delay, or it may instead be associated with absolute stunting of growth.
Fruit size comparison During
pregnancy, comparison of the size of the developing fetus to a fruit is a common practice. The fruits used as a reference vary by source. One source, the
Australian Department of Health describes an 8-week embryo as the size of a
raspberry, and a 21-week old fetus as the size of a
rockmelon. The Scottish
National Health Service describes an embryo at 5 weeks of gestation as the size of an "apple pip" and a
raisin at week 8. Nurses report these comparisons are accurate after measuring expected sizes with actual fruits. == Viability ==