Assessment of the first trimester
first-trimester pregnancy loss.
extraembryonic membrane that lines the chorion and contains the fetus and amniotic fluid.
consists of an outer trophoblast and an inner cell mass.
fetal heart rate below 90 beats per minute.
outermost of the fetal membranes; ultimately shrinks and is obliterated by the amnion between 12 and 16 weeks.
name applied to the endometrium during pregnancy.
portion of the endometrium on which the implanted conceptus rests.
decidua that covers the surface of the implanted conceptus.
decidua exclusive of the area occupied by the implanted conceptus; aka decidua vera.
composed of the decidua capsularis and decidua parietalis; thick hyperechoic rim surrounding a sonolucency; indicative of an intrauterine pregnancy.
term used for a developing zygote through the tenth week of gestation.
length of time based from conception.
gestational weeks 6 through 10.
visualization of the amniotic cavity without the presence of an embryo.
length of time calculated from the first day of the last menstrual period.
fluid-filled structure normally found in the uterus, containing the pregnancy.
refers to the number of times a woman has been pregnant including the current pregnancy, if applicable.
pregnancy located within the uterus.
solid mass of cells formed by cleavage of a fertilized ovum.
the sonographic appearance of subcutaneous accumulation of fluid behind the fetal neck in the first trimester of pregnancy; increases associated with chromosomal and other abnormalities.
refers to the number of live births.
centrally located endometrial fluid collection demonstrated with a coexisting ectopic pregnancy.
fetal heart rate exceeding 170 beats per minute.
provides nutrients to the embryo and is the initial site of alpha-fetoprotein.
Early embryology (fig. 23-1)
• Fertilization to implantation—approximately 5 to 7 days.
• Ovum and sperm join in the distal fallopian tube, forming a zygote.
• Cells of the zygote multiply, forming a cluster termed the morula.
• Fluid rapidly enters the morula, forming a blastocyst.
• After implantation—trophoblastic growth continues.
• Maternal vessels erode, establishing a circulation on the maternal side of the forming placenta (chorion basalis).
• Trophoblastic tissue covers the entire embryo, developing into the fetal side of the forming placenta (chorion frondosum).
• Human chorionic gonadotropin (hCG) is secreted by the trophoblastic tissue.
• Organogenesis is generally completed by the tenth gestational week.
Blastocyst development (fig. 23-2)
• Chorionic villi evenly surround the blastocyst.
• Embryo is located between the amnion and yolk sac (Fig. 23-2, A).
• Embryo folds into the amnion.
• Amnion attaches to the anterior portion of the embryo.
• Yolk sac becomes “pinched” near the embryo, forming the body stalk.
• Chorionic villi become more prolific near the implantation site (Fig. 23-2, B).
• Amnion begins to fill more of the chorionic cavity.
• Yolk sac is pushed into the chorionic cavity.
• Umbilical cord begins to develop about the seventh to eighth gestational week.
• Areas of the chorion away from the implantation site become smooth (Fig. 23-2, C).
• Amnion fuses to the smooth chorion.
• Embryo or fetus lies within the amniotic cavity.
• Chorionic villi and decidua basalis have formed a placenta (Fig. 23-2, D).
Anatomy
| STRUCTURE | DESCRIPTION | NORMAL SONOGRAPHIC FINDINGS |
| Abdominal wall | Physiological herniation of the fetal bowel into the umbilical cordBowel returns into abdomen, and herniation resolves by the eleventh gestational wk | Umbilical herniation contiguous with the umbilical cordAbnormal if persists after 12 wks’ gestation |
| Cardiovascular system | First system to function in the embryoFour heart chambers are formed by the eighth gestational wk | Cardiac motion as early as 5.5 wks |
| Cranium | Prosencephalon—forebrainMesencephalon—midbrainRhombencephalon—hindbrain | Prominent cystic space in the posterior portion of the brain (rhombencephalon) |
| Skeletal system | Vertebral bodies and ribs are forming at 6 wksArms and legs are forming at 7 wksOssification of the vertebral bodies and rib cartilage at 9 wksLong bones form during the tenth wk | Spine appears as parallel echogenic linear structures in the center of the embryo or fetusLong bones appear as hyperechoic linear structure(s) within the soft tissue of the extremities |
Laboratory values
Human chorionic gonadotropin (HCG)
• Produced by the trophoblastic cells of the developing chorionic villi.
• Normally doubles every 30 to 48 hours during the first 6 weeks of pregnancy.
• Peaks at the tenth gestational week (100,000 mIU/mL).
• Declines after the tenth week and levels out at about 18 weeks (5000 mIU/mL).
• Gestational sac should be identified transvaginally after the hCG levels reach 1000 mIU/mL and as early as 500 mIU/mL.
First-trimester measurements
Mean sac diameter (MSD)
• Establishes gestational age before visualization of an embryonic disc.
• Measures the length, height, and width of the inner-to-inner borders of the gestational sac.
Nuchal translucency
• First-trimester screening for chromosomal abnormalities.
• The gestation should be 11 weeks, 0 days, to 13 weeks, 6 days, and the crown–rump length (CRL) should be a minimum of 45 mm and a maximum of 84 mm.
• Midsagittal section of the fetus should be in a neutral spine-down position.
• Magnify so that only the fetal head and upper thorax should be included in the image.
• Maximum thickness of the subcutaneous translucency between the skin and soft tissue overlying the cervical spine is measured.
• Calipers are placed on the hyperechoic lines, not in the nuchal fluid, from the inner-to-inner borders perpendicular to the fetus.
• More than one measurement must be taken, and the maximum one is to be recorded.
• Nuchal translucency exceeding 3 mm is abnormal.
• Pitfalls include poor fetal position, maternal obesity, and mistaking the amnion for the fetal skin line.
Indications for sonographic evaluation
Sonographic Findings in the First Trimester
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| GESTATIONAL FINDING | DESCRIPTION | NORMAL SONOGRAPHIC FINDINGS | ABNORMAL SONOGRAPHIC FINDINGS |
| Gestational sac (GS) | Fluid-filled structure normally found in the uterus, containing the developing embryoFirst definitive sonographic finding to suggest early pregnancyAnechoic structure represents the chorionic cavityEchogenic rim represents decidual tissue and the developing chorionic villiBeta hCG of 1000 mIU/mL should demonstrate a GS transvaginally | Round anechoic structureSurrounded by a thick hyperechoic rim (2 mm)Located in the mid- to upper portion of the uterusEccentric location within the endometriumTransabdominal5 mm mean sac diameter (MSD) about 5-6 wksDouble decidual sign evident with an MSD of 10 mmTransvaginal2-3 mm about 4-5 wks | Irregular or distorted GSLarge GS without evidence of YSAbnormal uterine locationVisualization of amnion without concomitant embryoTransabdominalFailure to identify a YS with an MSD ≥20 mmFailure to identify an embryo with cardiac activity in a GS ≥25 mmTransvaginalFailure to identify a YS with an MSD ≥8 mmFailure to identify an embryo with cardiac activity in a GS ≥16 mm |
| Yolk sac (YS) | Located in the chorionic cavityProvides nutrition to the embryoEarliest structure visualized in the gestational sacAttached to the embryo by the vitelline ductUsed as a landmark to locate the embryonic disc and early cardiac activityUltimately detaches from the embryo and remains within the chorionic cavity | Hyperechoic ring within the gestational sacRound or oval in shapeInner-to-inner border diameter should not exceed 6 mmTransabdominalEvident within an MSD of 20 mmTransvaginalEvident within an MSD of 8 mm | YS diameter exceeding 8 mmTransabdominalFailure to identify a YS with an MSD ≥20 mmFailure to identify an embryo with cardiac activity in a GS ≥25 mmTransvaginalFailure to identify a YS with an MSD ≥8 mm |
| Embryo | Embryonic period extends from the sixth through the tenth gestational wks | Initially a local thickening adjacent to the yolk sacEchogenic focus adjacent to the yolk sacTransabdominalUsually detected within an MSD ≥25 mmTransvaginalUsually detected in an MSD of ≥16 mm | Embryo too small for gestational sacTransabdominalFailure to identify an embryo with cardiac activity in a GS ≥25 mmTransvaginalFailure to identify an embryo with cardiac activity in a GS ≥16 mm |
| Amnion | Initially surrounds the newly formed amniotic cavityAttaches to the embryo at the umbilical cord insertionExpands with accumulation of amniotic fluid and growth of the embryoObliterates the chorionic cavity by the sixteenth wk | Thin hyperechoic line between the embryo and the yolk sac (chorion) |