The Second Trimester: Weeks 13 - 24

December 14, 2007

If you were to map out nine calendar months, you would find that it counts out approximately forty weeks.  These forty weeks are what the obstetrician uses to date a pregnancy.  Since pregnancy and fetal development is a continuum toward the end result--a baby--there are many changes that occur on this long, strange trip.

At each stage of a pregnancy the growing baby is not the same baby he or she was weeks earlier.  Different considerations and concerns mark the different portions of the entire pregnancy, and historically gestation has been divided into three main sections--the first, second, and third trimesters.  While the first trimester is crucial for laying down the groundwork for the developing organs and concerns over the possibility of miscarriage, and the third trimester centers on the baby attaining maturity and the delivery process itself, the second trimester is a sort of reprieve during which the pregnancy can almost be enjoyed.

These weeks, weeks 13 - 24, are a time when a patient has the chance and peace of mind to really learn about having a baby. During this period, a woman has become somewhat accustomed to the strain on her physiology, her physical complaints usually decline, and the visits to her doctor are mainly to make sure that she's still happy, alive, and pregnant.

Of course this is a simplification.

Visiting The Doctor for a Pregnancy Check-Up

An obstetrician is actually doing a lot more during these "well-pregnancy" visits.  The fetal heart tones are assessed.  The size of the baby is estimated and compared to the sizes recorded on previous visits to assure appropriate interval growth.  Ultrasound is utilized to diagnose only a single baby versus twins (or more!). The minor complaints are addressed, the doctor especially sensitive to what might be a major problem presenting as a minor complaint. The lab work is monitored to watch for anemia, urinary tract infections, or warnings of pregnancy-related illnesses, like toxemia (later named pre-eclampsia, then later re-named pregnancy induced hypertension).

In evaluating size, some doctors actually measure the height of the womb (fundal height of the uterus) as felt through the prospective mother's skin.  An old Charity Hospital cheater way to do it was to assume this fundal height reached the belly button (umbilicus) at 20 weeks, and was either a week more for every inch above it (about the doctor's finger width) or a week less for every finger counted below it.  In other words, if the fundal height were two finger-widths above the belly button, this would correspond to 22 weeks.  This was helpful for those patients who used to roll in with absolutely no prenatal care and we had to make a determination (albeit crude) on how far pregnant they were. In my experience, it wasn't any more inaccurate than measuring with a tape measure.

Pregnancy Problems

Two major problems of note during the second trimester are the incompetent cervix and placental abruption. Around 20 weeks is the most famous "fright zone" for these complications.  An incompetent cervix just doesn't have the strength to stay closed any longer, dilating painlessly (without contractions) leading to a premature delivery. Since prematurity is one of the most costly medical complications there are, any patient at risk should be evaluated during this high-risk time.(Even when a cerclage--a closing noose surgically placed around the cervix--has been used, this is the time when a baby has attained a size to put stress on it.)  Sometimes prolonged bed rest is necessary as well.

The placenta (after-birth) has usually picked a permanent spot during the second trimester, the rest of the uterus expanding away from its site of implantation.  But if that site is over the exit route of the baby (over the cervix--on the inside, called the "os"), this not only throws a block for the only natural way out at delivery, but also sits as an endangered highly vascular organ that can tear away ("abruption") as the cervix thins and cause hemorrhage that endangers both mother and child.  This abnormal placement of the placenta is called a "placental previa," or "coming before."  The second trimester is the usual time for a previa to tear away (to abrupt), and the timing couldn't be worse.

It used to be that 28 weeks was considered the "time of viability," i.e., that gestational milestone after which a premature baby could survive.  With the advent of neonatal ICUs, however, this time has been whittled down.  Currently, 25-weekers are entertained as survivable.  Less than that would put a baby into second trimester territory, but this is a zone that is hard to cross when talking about a baby surviving in the outside world.  Ultrasound has given many life-saving warnings for who is at risk for placenta previa, and follow-up ultrasound has been helpful in either admonishing those women who continue in peril or in reassuring those whose risk lessens over time.

Pre-eclampsia is typically a third-trimester problem, but it is possible to have it develop earlier.  This early presentation is a sign that the pre-eclampsia will become fairly severe, and extremely close follow-up is mandatory.
The menstrual-like cramps of the growing pains of the first trimester are replaced by the second trimester's "round ligament" pains.  The round ligaments are supports which originate on the sides of the uterus, run through the inguinal canals, and insert on the side walls of the vagina and even inner thigh.  It is during the second trimester when the uterus gets big enough so that it can no longer sit in the pelvis but pops up to become an abdominal organ.  Since the round ligaments are a pretty lousy support even when the uterus is only the size of a pear, they're even a nuisance when the uterus gets considerably larger in the second and third trimesters.  The uterus can actually fall from side to side, twanging the round ligaments so that a woman complains of pain on one or both sides where her hip flexes up against her abdomen. This is the inguinal region where the round ligament is crossing. It is a completely harmless pain, but is can really knock a woman for a loop and stop her dead in her tracks. Usually, changing position will relieve this strain on the ligament, lessening the discomfort, but the only final cure is delivery several months later.

The second trimester is the first time real expansion begins.  Stretch marks begin, sometimes invisibly.  Everything grows!  Moles get bigger, skin tags get bigger, even warts can get bigger.  Thanks to estrogen, everything has a better blood supply.  This also explains why gums may bleed after tooth brushing.  This is also the most stable time for the pregnancy, too.  If any surgery need be done, this is the safest time--too late to provoke miscarriage, too early to stimulate premature labor.

The second trimester gives a pregnant woman a break, for the first and third trimesters provide much more intense worries.  And if the big, bad things are ruled out, this time can afford a woman the opportunity to rejoin the world after the initial frights of the first 12 weeks.

For more information on fetal development check out our pregnancy videos.

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