Menstrual Age versus Gestational Age: Menstrual age refers to the first day of the last normal menstrual period preceding pregnancy. Clinically, the last menstrual period is used for pregnancy dating because it is an identifiable landmark (whereas, conception usually is not). Gestational age is based on age from conception and therefor usually lags menstrual age by 2 weeks. In reality, most clinical texts use the two terms interchangeably while ìpuristsî will use the terms correctly.
First Trimester The first trimester of pregnancy is defined as the thirteen weeks following the first day of the last
menstrual period (LMP). This definition is based on the time in which the gravid uterus has not yet
ìrisenî from the pelvis. The first ten weeks are considered ìembryonic periodî and comprise the time
when organs are forming. Weeks 11-12 are ìfetalî. A full term pregnancy is 40 weeks long.
Embryology: Basic embryologic events are reviewed in Table 1.
Table 1: Embryology of Early Pregnancy
Ovarian follicle matures
The first trimester of pregnancy is arguably the most exciting time in human development. However, the
first trimester is also an emotion-charged time because this period is fraught with a high failure and
complication rate. Ultrasound, specifically transvaginal ultrasound technique, allows for the best
visualization of first trimester structures and provides the best opportunity to detect abnormalities in the
GSD= Mean Gestational sac diameter TV = Transvaginal US TA = Transabdominal US CRL = Crown Rump Length
Gestational Sac : The first structure seen with ultrasound, the gestational sac can be visualized as
early as 4.5 weeks by transvaginal technique. A normal gestational sac is round or oval and is located
within the fundus or mid portion of the uterus. Echogenic (bright) borders and an eccentric position
within the endometrial cavity (the double decidual sac) help differentiate a "true" gestational sac from a
ìpseudoî gestational sac (seen with ectopic pregnancy).
Yolk Sac: The yolk sac is the first structure visible within the gestational sac. The yolk sac should
always be seen when a gestational sac measures greater than 10 mm. A normal yolk sac is round and
measures less than 6 mm. If the yolk sac measures greater than 6 mm, is bizarre in shape or is calcified,
follow up exam is indicated since most pregnancies with abnormal yolk sacs will fail.
Embryo: The second structure that becomes visible within the gestational sac is the embryo.
Embryonic cardiac activity should always be seen when an embryo measures greater than 5 mm.
With improvements in ultrasound instrumentation, it now is possible to evaluate some embryonic and
fetal anatomy during the first trimester. Between 8 and 10 weeks, the rhombencephalon, a fluid filled
cavity in the hindbrain, is easily identified. This chamber eventually becomes the fourth ventricle and
central canal of the brain and spinal cord. Normal physiologic bowel herniation is also routinely seen
before 14 weeks.
Membranes and Placenta: The amniotic cavity, chorionic cavity, and the site of placentation
are well demarcated by transvaginal ultrasound. The amnion grows against the chorion and the
membranes eventually fuse, usually completely by 17 weeks.
In order to adequately assess a first trimester pregnancy, the gestational sac (GS) size or embryonic
crown rump length (CRL) should be compared with the menstrual age. The GS should be measured in
three orthogonal planes and the mean sac diameter (MSD) calculated by averaging these measurements.
The equation, MSD + 30 = menstrual age (days), can be applied (first eight weeks) when a chart of
sac size is not easily accessible.
The most accurate sonographic measurement correlating with menstrual dates is the CRL. This
measurement is the maximum visible length of the embryo. Care must be taken not to include the
umbilical cord or yolk sac in the crown rump measurement. Between six and ten weeks there is little
variability in embryonic size, with the CRL = menstrual age in weeks +/- 4-5 days.Every ultrasound
department should choose and follow tables which correlate menstrual age with MSD and CRL
measurements. Embryonic CRL measurement is more accurate than using LMP dates or physical exam
in dating a pregnancy.
ROLE OF HCG: Serum quantitative human chorionic gonadotropin (HCG) may be correlated with
gestational age. Different HCG preparations are in use that have different normal values. The two most
common are the first International Reference Preparation (first IRP) and the Second International
Standard, (2 IS). The first IRP values are approximately two times those of the 2 IS. It is important to
be familiar with the preparation being used at your hospital.
An important use of serum HCG levels in the first trimester concerns the discrimination between a
normal intrauterine pregnancy and an abnormal pregnancy that may be ectopic. To this end, the so-
called discriminatory HCG level has been defined. If the serum HCG concentration exceeds the
discriminatory level of 800-1,000 units/liter (2 IS) and 1,000-2,000 units/liter (first IRB), then an
intrauterine gestational sac should be visible with transvaginal sonography. If the HCG values exceed
these levels and an intrauterine gestational sac is not visible, then ectopic pregnancy or recent
spontaneous abortion are possible. However, It is prudent to set ones own "discriminatory levels".
Pregnancy failure is a common problem in the first trimester with failure rates approaching 25%. A
threatened abortion is defined as bleeding and cramping in the first 20 weeks of a pregnancy.
Ultrasound plays a key role in evaluating women with threatened abortion since HCG levels do not
correlate well with a specific diagnosis. Familiarity with normal first trimester landmarks (Table 2) is
essential in order to diagnose a failing or nonviable pregnancy.
Perigestational Hemhorrage: Perigestational hemorrhage, from the chorionic frondosum
(early placenta) is the most common source of vaginal bleeding during normal intrauterine pregnancy,
and up to 20% of women with a threatened abortion have a subchorionic hematoma. Large
perigestational hemorrhages have been associated with pregnancy loss while smaller perigestational
hemorrhages usually resolve without sequelae.
Anembryonic Pregnancy (Blighted Ovum): Anembryonic pregnancy is a form of failed
pregnancy defined as a gestational sac in which the embryo failed to develop. The use of the term
"blighted ovum" is discouraged. A large gestational sac without a visualized embryo is unequivocal
evidence of a failed, anembryonic pregnancy.
Embryonic Demise and Bradycardia: The most convincing evidence that a pregnancy
has failed is the documentation of embryonic demise. As stated previously, all embryos greater than 5
mm should demonstrate cardiac activity. Embryonic demise should be diagnosed only after careful
observation by two experienced people, preferably sonographer and physician, for at least 3 minutes.
Embryonic bradycardia is a poor prognosticator of pregnancy viability and needs follow-up. An
embryonic heart rate less than 90 beats per minute, in embryos less than 8 weeks, is associated with an
80% rate of eventual embryonic demise.
Poor Growth: First trimester growth retardation is a sign of a failing pregnancy. Growth
retardation is easily detected by comparing the MSD with the CRL or by following the growth of these
parameters serially. The early gestational sac grows approximately 1mm per day.
The most reassuring sign that an ectopic pregnancy is not present is the sonographic demonstration of a
normal intrauterine pregnancy. The presence of an intrauterine pregnancy decreases the risk of a
concurrent ectopic pregnancy to 1 in 30,000 for a low risk patient and 1 in 5,000 for a high risk patient
(history of pelvic inflammatory disease (PID), previous ectopic, infertility, tubal surgery). Transvaginal
ultrasound, with a reported accuracy of greater than 90%, should routinely be used in the evaluation for
A variety of uterine findings can be seen with ectopic pregnancies. The uterus may be empty or contain
endometrial fluid collection (called pseudo-gestational sac seen in 10-20% of cases). This should not
be confused with an intrauterine gestational sac.
The most common adnexal (tubal) finding in the presence of an ectopic pregnancy is a complex adnexal
mass which represents hemorrhage. Other adnexal findings include a normal adnexa or a well formed
adnexal "ring" with or without a yolk sac or embryo. Hemorrhage, either within a fallopian tube or
peritoneal space, may be the only sonographic finding at the lowest HCG levels. The posterior uterine
cul-de-sac (Pouch of Douglas) should be carefully investigated since complex peritoneal fluid may be
the only finding in 15% of ectopic pregnancies.
An exophytic ovarian corpus luteum can mimic an ectopic pregnancy. However, the corpus luteum
usually contains more echoes than an adnexal ring and is demonstrated as intra-ovarian by transvaginal
imaging. The majority of ectopic pregnancies. are on the same side as the corpus luteum and we
routinely try to identify the corpus luteum in our ectopic searches. Doppler ultrasound may help
differentiate luteal flow from trophoblastic flow.
Careful investigation of the uterus and adnexa is recommended as part of the routine first trimester
evaluation. Myomas (fibroid) can grow during pregnancy and if they are located in the lower uterine
segment, they can obstruct the birth canal. Uterine duplication anomalies are common and are
associated with an increased risk of pregnancy loss.
In the first trimester, molar pregnancies have variable sonographic appearances. Intrauterine findings
range from the presence of an anembryonic gestational sac to an endometrial mass with or without
cystic areas. Other findings associated with GOD include perigestational hemorrhage and ovarian theca
lutein cysts. Theca lutein cysts occur in 50% of molar pregnancies and are thought to be secondary to
increased levels of circulating HCG. Sonographically these cysts are large, bilateral, and multi septated.
While we can suggest the presence of GOD, it is only the pathologist that can definitively make this
With the advent of endovaginal high frequency transducers and better ultrasound technology, detailed
early evaluation of embryonic anatomy may soon be possible. We will review some normal embryologic
anatomy as well as some anomalies we have been able to detect in the first trimester. This use of
ultrasound is still experimental but the future of anomaly detection may be in the first trimester.
The pregnant patient is best served by a confident and caring physician who vigilantly
searches for high-risk features and then treats the patient as is appropriate for each high-risk
condition. Most patients have completely normal pregnancies, but the high-risk pregnancy
mandates changes in the "normal" evaluation of the pregnant patient.
The process of antenatal care is on-going risk assessment:
What is the genetic risk? (maternal age, abnormal MSAFP screening, folate
administration for prior NTD)
What is the risk for preterm birth? (BV screening, history of preterm birth)
What is the risk for pregnancy-induced hypertension? (history)
What is the risk for IUGR? (past history, small uterine size for dates)
What is the risk for blood group isoimmunization? (Rhogam for Rh- women NOT
After the core curriculum, using this lecture as a basis, you should be able to:
List at least five conditions identified by medical history that results in a high-risk
List at least six conditions found during physical examination of a pregnant woman that
denotes a high-risk pregnancy.