As we move into the second trimester,
we're gonna be going through weeks 13, 14, 15, 16, 17 and 18, during month four.
In this period of time, the elongation of the embryo is very rapid
and we move from 7.4 cm up to approximately 14.2 cm.
As we're proceeding through this time, growth is incredibly rapid,
the face is gonna start having the eyes, face anteriorly and we're gonna have the ears
reaching their more or less mature position from the lower down on the neck.
Coordinated limb movements and eye movements
are going to be visible at this time
and grasping motions may be seen on ultrasound.
The ovaries are distinct and contain primary follicles by week 16,
and the ossification of the long bones and facial bones is going to be continuing
and those centers getting more and more elongated.
Feel of movements are often felt by the mother during week 17.
As we move into the next month, we're gonna have weeks 19, 20, 21, and 22 occur
and we're gonna lengthen from approximately 15.3 to 27.8 cm.
This elongation is a little bit slower than it was in the prior months.
So month 5 is still undergoing rapid growth
but not quite as quick as compared to month four.
At this time, the vernix caseosa is present on the surface of the body
as well as the lanugo hair that helps keep it in contact with the fetus
and protect it from irritation by the amniotic fluid.
In week 20, the testes are present but are not yet in the scrotum.
They're retroperitoneal and are beginning to descend through the anterior body wall.
The eyebrows and head hair are going to be present at this time,
growing into the head as opposed to the lanugo hair,
which will fall out and not persist after delivery.
In week 21, we have the uterus formed
and the vagina has been canalized, the vaginal plate has opened.
Rapid eye movements are detectable in week 21 as well, and by week 22,
hair on head and eyebrows are not just present but can get thicker and more pronounced.
As we move into month six, midway through week 23,
we're going to lengthen a bit more from 28.9 cm to 36.6 cm.
Again, growth is still occurring, but not quite rapid as it had during month number four.
Type II pneumocytes begins secreting surfactant as early as week 23,
but this surfactant is not gonna be present in enough quantity
to allow easy inflation of the lungs should the fetus be born this prematurely,
and the flatten pneumocytes
are probably not capable of generating a great deal of gas exchange
between the fetal circulation and the fetal airway.
So at 23 weeks, we see surfactant beginning to be produced
but it's not yet enough to accommodate
the needs of the growing fetus outside the womb.
By the 26th week the eyelids have reopened
and by the 27th week, we do in fact have enough surfactant there
to potentially allow the lungs to expand and not collapse
and there is potentially flat enough pneumocytes
in contact with the airway and the fetal circulation
through the lungs, that gas exchange can occur.
And this 27th week is very important
for the reason that it's the earliest we can hope
for a premature infant to survive outside the womb with supportive care.
In the 26th week, white fat has begun to accumulate in the body
and it accounts for roughly 3.5% of body weight.
And that percentage will only increase as development proceeds throughout the fetal period
unless we're carrying an infant far past term at which point it will start to decrease.