How do doctors determine how many months you have been pregnant?
I admit that i am not an Obstetrician but my basic medical
training has equipped me fairly well enough to answer this one.
Doctors employ a variety of methods to ascertain the gestational
age of the fetus or the duration of pregnancy. (These terms are
often used interchangeably although in some circumstances, they may
convey slightly different meanings.) Traditionally, the duration of
pregnancy is calculated from the date of the last menstrual period
(LMP.) Once the diagnosis of pregnancy is established by whatever
reliable means, the duration of pregnancy is calculated by simply
counting the days from the first day of the last menstrual period.
For a woman with regular menstrual cycles or one who keeps track
with a calender, getting an accurate date may not pose a problem.
For many others only an approximate value can be obtained by this
method. A pitfall with this method is the fact that some pregnant
women may actually bleed in the early months of an otherwise
uneventful pregnancy and mistake this often ominous sign for her
regular menses. Thus the calculations may be inaccurate by as much
as two months. Clinicians also rely on their findings on physical
examination. The uterus increases in size at a fairly predictable
rate during pregnancy. The fundus, or the top of the uterus can be
felt the level of the pelvic brim (bony ridge at the lower abdomen)
at thirteen weeks (approximately three months) of pregnancy. The
fundus then rises approximately one centimeter with each passing
week. It reaches the umbilicus at roughly twenty eight weeks and
the lower margin of the rib cage at thirty six. It then drops
slightly over the next two weeks as the fetus descends into the
pelvis. These methods may seen crude and unsophisticated but are
surprisingly quite reliable regardless of the mothers physical
stature and afford a quick and easy means of crosschecking
calculated values with estimates from physical examination
(provided you have a single fetus; with twins and triplets all
calculations are off.) Ultrasound scans have long since
revolutionized the monitoring of pregnancy. The ultrasound creates
an image of internal parts as high frequency sound waves are
reflected from tissue surfaces. The ultrasound scan can detect the
gestational sac(or sacs), its location, size, the fetal heart beat
(in viable pregnancies), the fetal size, location of the placenta
etc etc. The ultrasound scan is considered fairly safe although its
use is not without problems. Using the ultrasound scan, the
clinician can record the dimensions of particular fetal structures
at different stages of pregnancy eg. Crown rump length, biparietal
diameter, femur length,etc (body length, head size, leg length
respectively.) These values are compared with tables and graphs
obtained from studies of fetuses in many pregnancies that were
accurately dated to obtain the duration of the pregnancy in
question. The values thus obtained are more accurate the earlier on
in pregnancy they are obtained because of the smaller margin of
associated error. As pregnancy progresses the estimated range
widens. I hope this has been helpful. Thank You
