First Trimester Obstetrical Ultrasound

At the conclusion of this course, you will be able to: Discuss embryonic development Discuss the goals of a 1st trimester scan Explain the measurements that are taken during a first trimester scan Explain ectopic pregnancy and its sonographic findings Explain clinical presentation of ectopic pregnancy Describe first trimester screening techniques for early fetal abnormalities   The Role of Ultrasound in the First Trimester Kristina Cabello   Pregnancies are dated starting with the first day of the last menstrual period (LMP).   Regular menstrual cycle of 28-30 days will typically conceive 2 weeks after the start of the last menstrual period.   Gestational age is a term that is used to classify the date of a pregnancy.   This is conception age plus 2 weeks.    For women with regular menstrual cycles of 28-30 days, the gestational age and menstrual age are the same.   Determining gestational age when a patient first presents is difficult, not to mention highly inaccurate.   Clinical dating relies on: Accurate historical dates by the patient Regular menstrual cycle Accurate conception date Clinicians rely on: Patient to accurately know their last menstrual period Uterine sizing by physical exam   Uterine size for clinical dating can be inaccurate due to patient body habitus and the presence of uterine fibroids.    If uterine size is indeterminate or LMP is unknown or not reliable, ultrasound can be very useful and extremely accurate in estimating gestational age. Timing of chorionic villus sampling in the 1st trimester   Genetic amniocentesis during the 2nd trimester   3rd trimester induction or cesarean delivery decisions   Biochemical screening accuracy   To make distinction between term and pre-term labor   To characterize a fetus as post dates   Determine normal versus abnormal fetal development The 1st trimester of a pregnancy is identified by numerous quick transformations spanning a time period from fertilization through the weeks of early fetal life at 11 weeks, 6 days.   It is identified by several time periods of embryonic development.   These time periods consist of the following: Fertilization Blastocyst formation Implantation Gastrulation Time periods of embryonic development:   Neurulation Embryonic period Early fetal period During the first two weeks of the menstrual cycle, the pituitary gland excretes follicle stimulating hormone, FSH, and luteinizing hormone, LH.    These hormones cause ovarian follicles to enlarge and secrete estrogen.    During this time, one follicle will become dominant and continues growing until the time of ovulation.    Estrogen is low until about 4 days before ovulation.    At this time the dominant follicle produces a surge of estrogen.    After this surge of hormone, LH and prostaglandin surge causing ovulation.   Around day 14 of the menstrual cycle, the secondary oocyte, which originates from the dominant follicle, expels from the ovary in what is defined as ovulation.    The follicle collapses to form the corpus luteum.    The corpus luteum secretes progesterone and some estrogen.    Due to the progesterone secretion, the endometrium becomes thickened in anticipation of potential implantation.    When pregnancy occurs, continued progesterone secretion causes thickening of the endometrial lining.  This is defined as a decidual reaction. On Day 14, fertilization occurs when the ovum and the sperm come together to form the zygote.    As the zygote travels through the fallopian tube, the cells continue to divide.    On Day 18, when the cells have divided to the 12-16 cell stage, it is now called a morula and enters the uterus. On Day 20, it’s developed to the blastocyst stage.     A blastocyst is a fluid-filled cyst lined with trophoblastic cells that contains a cluster of cells on one side called the inner cell mass.  On Day 20, implantation occurs.    This usually takes place at the fundal region of the uterus. After implantation occurs, the primitive yolk sac forms but is shortly thereafter pinched off resulting in the secondary yolk sac.    This is formed at around Day 27 or 28 when the mean gestational sac diameter is about 3 mm.    The secondary yolk sac is what we visualize by ultrasound. During Week 5, through a process called gastrulation, cells change from a bilaminar disk to a trilaminar disk.   Trilaminar disk consists of three primary germ cell layers:  Ectoderm Mesoderm Endoderm Ectoderm:  This gives rise to the outermost skin layer, central and peripheral nervous systems, eyes, inner ear, and other connective tissue.   Mesoderm:  This comprises the primitive heart and circulatory system, the foundation for bones, muscles, kidneys, and reproductive systems.   Endoderm:  Will develop into the lungs, intestines, and bladder. Neurulation, which is the process of the formation of the neural plate and its closure to form the neural tube, begins at week 5.   It starts with the thoracic level and extends cranially and caudally until completely closed by the end of week 6. During week 5, two cardiac tubes are formed.    By the end of week 6, blood flow is unidirectional.    By the end of the 8th week, the heart has definite form.  By the end of the 5th week, the primitive embryonic vascular network is formed.   The umbilical cord is formed at the 6th week.    The umbilical cord contains 2 umbilical arteries and 1 umbilical vein.   The peripheral vascular system takes longer to develop but it is complete by the end of week 10. The limb bud formation takes place during the 5th week.   At week 6, the primitive gut forms and from the 8th – 12th weeks, the mid gut herniates into the base of the umbilical cord.    The central nervous system begins forming with the primary brain vesicles at 6 weeks.    At the 8th week, the rectum separates from the urogenital sinus and the anal membrane perforates by the end of the 10th week.   The primitive kidneys begin to ascend to the pelvis at week 8; by the 11th week they are located in their adult position. By the end of week 9, the lateral and 3rd ventricles are recognizable.   By the end of the 9th week, the limbs are formed with distinct fingers and toes.   External genitalia remain in the sexless stage at the end of week 10.    External genitalia will reach their mature fetal form by the end of the 14th week. The embryologic period comprises the time period beginning with week 7 and ending with week 10.   By the end of 10 weeks, the embryo measures just a little over 30 mm in length and weighs about 4 grams.   Beginning with the 11th week, the fetal period begins and goes through 40 weeks.    The head makes up nearly half the size of the fetus at this early fetal stage.    During the fetal period, the fetus’ body growth is quite rapid whereas the head growth somewhat slows. Localize the gestational sac   Identify embryonic viability   Estimate menstrual age   Assess the number of fetuses   Early detection of fetal abnormalities   From 5 weeks until the end of the 1st trimester, ultrasound for gestational dating is generally done at a high accuracy.   There are certain sonographic findings by date that can be used to confidently assign a gestational age.   Early intrauterine pregnancy can be visualized by 5 weeks menstrual age.   The presence of an intrauterine pregnancy can be identified by: 5 weeks transvaginal 5.5 weeks transabdominal  This is visualized as a round or oval fluid collection.  The intradecidual sign will appear as focal endometrial thickening with a small gestational sac.   This is also about the time that a pregnancy test will show up positive. Early 1st Trimester Gestation The double decidual sign, which will appear as the 3 layers of decidua, comprises: A ring formed by chorion frondosum and decidua basalis at the implantation site Chorion laeve Decidua capsularis   The double decidual sign is not always seen in all cases.    It may only be seen in about 1/3 of cases.    Not seeing this sign is not necessarily an indication of prognosis or of positive or negative pregnancy outcome.    If this is not seen, consider it of non-clinical importance. Early 1st Trimester Pregnancy Yolk sac will be the first structure to be visualized within the gestational sac.    With endovaginal scanning, this should be visualized with a Mean Sac Diameter (MSD) of 8 mm.    With transabdominal scanning, this is typically visualized when the Mean Sac Diameter is approximately 10 to 15 mm.  MultiSlice Imaging with Zoom MPR Early Yolk Sac Seeing a yolk sac within a gestational sac is 100% proof of an intrauterine pregnancy.    Early on in the pregnancy, the yolk sac grows rapidly but then begins to slow as it reaches its max size of 5-6 mm between 5-10 weeks menstrual age.    In the early stage of pregnancy, the yolk sac is pivotal to the embryonic development.    The yolk sac’s role is the transfer of nutrients.   The number of yolk sacs will coincide with the number of fetuses.  Normal twin pregnancies can result from one yolk sac. 3-ScapeTM Real-time 3D Imaging 1st Trimester Fetus and Yolk Sac By transvaginal scanning, as early as 5 weeks or when the CRL measures 2 mm, the embryonic disk will be visualized between the yolk sac and amnion.    Cardiac activity seen:  transvaginal 6 weeks transabdominal 6.5 weeks   Detect embryo 5 mm or larger with cardiac activity at 6.3 weeks 7 week Embryo - Fetal Heart Rate These zoomed-in 2D views of a 1st trimester embryo shows the presence of cardiac activity.  As stated earlier, this helps us to prove the existence of a viable pregnancy. 10 week Embryo Cardiac Activity 10 week Embryo Cardiac Activity   8 Week Embryo Cardiac Activity 8 Week Embryo Cardiac Activity   The amnion will be visualized as a thin membrane.    It’s difficult to visualize the amnion as fetal renal development continues and urine production takes off by week 9.   The amniotic cavity extends to fill the chorionic cavity by the 14th – 16th weeks.   Approximately 5 cc of fluid is produced per day at 12 weeks menstrual age.  7 week Embryo - Amnion Rupture of the amniotic membrane is rare but could result in amniotic band syndrome.    This may cause portions of the embryo to stick to the amniotic band or swallow parts of it resulting in a broad variety of abnormalities. You can use the Mean Sac Diameter to estimate menstrual age from 5 weeks to 10 weeks.    The accuracy of this measurement during this time period is within approximately 1 week of menstrual age.     The Mean Sac Diameter is obtained by an average of 3 sac diameters: (Long, transverse, and anterior-posterior diameters) divided by 3.  Longitudinal Uterus Transverse Uterus By the time the Mean Sac Diameter measures 8 mm or greater, the sac should contain a yolk sac.    By the time the sac measures 16 mm or greater it should have an embryo within it.    These normal signs should be followed closely if not visualized as this may be an indication of impending failure. At 6 weeks menstrual age through the end of the 1st trimester, we will use Crown Rump Length (CRL) for gestational dating.    CRL is a measurement of the length of the embryo or fetus from the top of the head to the bottom of the torso.   CRL should exclude the yolk sac and the extremities.   The accuracy of the CRL is accurate to about 5-7 days.    During weeks 11 - 14, the CRL will double in size.    Up to 12 weeks the CRL is very accurate in dating the gestation.    From 12 weeks and beyond, the CRL is much less reliable.    Due to fetal flexion and extension the CRL is not as accurate by the end of the 1st trimester. 10 Week Embryo - Crown Rump Length Caution!  One pitfall with CRL, especially with older fetuses, is that measurements can be underestimated if the spine is flexed or overestimated if the spine is straight.   Use the Biparietal Diameter (BPD) at the end of the 1st trimester to obtain menstrual age as it becomes more accurate than the CRL during this time. 12 week Fetus - Flexion  12 week Fetus - Extension One of the main goals of the 1st trimester is to determine embryonic viability.    Utilizing ultrasound, we can detect early pregnancy failure.    In general, approximately 75% of all pregnancies will fail.    Many pregnancies spontaneously abort prior to or shortly after a missed period   This may be due to the following reasons: Fertilized ova fails to divide Loss prior to, during, or after implantation Luteal Phase Defect:  This occurs when the corpus luteum does not properly support the conceptus following implantation  There are still many unknown reasons that lead to pregnancy failure   The presence of cardiac activity is the most important sign to confirm embryonic viability.    Using the transabdominal approach, when the embryo is visualized, the cardiac activity should be demonstrated.   Although positive pregnancy outcome is ruled out by the presence of cardiac activity, there are other secondary signs that can also be useful in predicting a negative outcome: Presence of vaginal bleeding Subchorionic hemorrhage can be associated with embryonic mortality 12 week Fetus - Extension Embryonic bradycardia:  An abnormally slow heart rate can be an indication of impending demise.  In an embryo with a CRL less than 5 mm, a normal heart rate will be above 100 bpm.   Yolk sac: Since the yolk sac is the first structure that should be visualized within the gestational sac, endovaginally the sac must contain the yolk sac by 8 mm and transabdominally by 20 mm.    A yolk sac diameter greater than 5.6 mm between 5 and 10 weeks is usually associated with poor outcome.    An abnormally large yolk sac can be one of the first signs of poor outcome.  This can be associated with certain abnormalities such as Trisomy 21 or molar pregnancy.    A calcified yolk sac is an indication of embryonic demise.  Calcification usually takes place within 36 hours after demise.   Gestational sac appearance can give way to predicting poor outcome.    A normal gestational sac will have 1 mm of growth per day.    The following secondary sac characteristics regarding gestational sac can be used to determine pregnancy outcome and impending failure: When the embryo is not visualized inside the gestational sac Distorted gestational sac shape Weak echogenic trophoblastic cells   Low implantation position in the endometrium   Very large gestational sac without a yolk sac or embryo within it This transabdominal image shows a very small gestational sac for the expected clinical gestational age.  Notice some of the signs mentioned in the previous slides where there is no visualized embryo.  As a matter of fact, we can also observe the appearance of weak trophoblastic cells. Pregancy Failure Pregancy Failure   Ectopic pregnancy is one of the leading causes of maternal death.    According to the most current data from the Centers for Disease Control (CDC), ectopic pregnancy accounts for approximately 9% of all pregnancy related deaths.    The occurrence of ectopic pregnancy is 2%.   The classic clinical presentation of patients with ectopic pregnancy is: Palpable adnexal mass Pain Abnormal bleeding   When patients present with a positive pregnancy test, the 1st trimester scan must locate the presence of an intrauterine pregnancy.    Sonographically, the presence of a live embryo in the adnexa is a sure sign of ectopic pregnancy. Some of the secondary signs are:   Empty uterus   Pseudogestational sac - Visualizing the double decidual sign could help with this   Ascites or posterior cul-de-sac blood clots   Adnexal mass:  A positive pregnancy test and an adnexal mass cannot exclude the presence of an ectopic pregnancy   Ectopic tubal ring:  concentric ring created by trophoblast that surrounds the chorionic sac Approximately 95% of all ectopic pregnancies occur in the isthmus portion of the fallopian tube.    About 2-3% of ectopics occur in the intramural portion of the tube.  This is the portion of the tube that enters the endometrial canal.    Other locations are rare but could occur in the abdomen, ovarian, and cervical sites. The most devastating location for an ectopic pregnancy to occur is in the corneal location, otherwise referred to as an interstitial ectopic pregnancy.   Those located here often rupture later because they have slightly more space to grow and tend to present clinically later.    These often cause intraperitoneal hemorrhage from dilated arcuate arteries and veins.    The risk of mortality from this type of ectopic is two times as much as any other tubal ectopic. The medical management of ectopic pregnancy is typically the surgical approach.    Some success has come about with a less invasive medical approach with the advances in endovaginal scanning technology.    Surgical resection of the fallopian tube has been done to remove an ectopic pregnancy in the past. Advances in laparoscopic procedures have allowed for resection of the ectopic only.    This approach strives to preserve the fallopian tube so that successful intrauterine pregnancy can occur at a later time.    In very early detected ectopics, drugs such as Methotrexate can be administered to kill the rapidly dividing cells.    These cells are then reabsorbed to leave the fallopian intact.  Beta-HCG levels can be monitored as the levels drop to follow the ectopic pregnancy. Although fetal anomalies should be diagnosed during the 2nd trimester, advances in image quality of ultrasound systems have enabled us to obtain high quality 2D imaging of the nuchal fold.   80% of newborns with Down Syndrome present with redundant nuchal skin folds.   Measurements of the nuchal translucency, and the absence of the fetal nasal bone along with 2 maternal serum markers (free beta-HCG and PAPP-A), help to have an approximately 90% detection rate for Down syndrome. 1st Trimester Nuchal Translucency The most common ovarian mass visualized is a corpus luteum cyst.    The corpus luteum develops post ovulation to support the early pregnancy by secreting progesterone.    They are typically 5 cm during pregnancy and are usually a thin-walled, unilocular cyst. Corpus Luteum Cyst If cystic hemorrhage occurs, echogenic debris or internal septations can be seen within the corpus luteum cyst.    Any atypical size or appearance of a corpus luteum cyst should be followed up on subsequent exams. Twenty to thirty percent of women over 30 years old have uterine fibroids.    It is the most common neoplasm of the uterus.  They are even more commonly found in African-American women.   Fibroids vary in sonographic appearance.    Fibroids are solid masses but could appear calcified or contain focal cystic areas of necrosis.    Most fibroids will not change throughout the course of a pregnancy; however, since they are estrogen dependent, they may increase in size rapidly due to this influence. You should now be able to: Discuss embryonic development Discuss the goals of a 1st trimester scan Explain the measurements that are taken during a first trimester scan Explain ectopic pregnancy and its sonographic findings Explain clinical presentation of ectopic pregnancy Describe first trimester screening techniques for early fetal abnormalities