OB Midterm Review
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What are the indications for a 1st trimester ultrasound exam?
- 1) confirmation of pregnancy
- 2) presence of yolk sac
- 3) position of gestational sac (not ectopic, etc.)
- 4) presence of fetal heart rate
- 5) number of embryos
- 6) estimation of gestational age
- positive pregnancy test
- missed menstrual period
- Development, growth and well-being of the fetus.
- Must image the following:
- Sag UT w/meas.
- Trv UT w/meas.
- both ovaries in Sag & Trv
- gestational sac in Sag & Trv
- yolk sac
- crown rump length-longest image possible
- posterior cul-de-sac (should be no more than 2-5 ml. of fluid)
- both adnexa in sag & trv
- look for double bleb sign (amniotic cavity & yolk sac)
- fetal heart rate (90-200 normal range)
- (TV: 5-5.5 wks., TA: 6-6.5 wks.)
- 170 by 9 wks.
- 120-160 for duration
- <8 wks.=embryo
- >8 wks.=fetus
What are the indications for 2nd & 3rd trimester ultrasound exams?
- 3 most important findings:
- fetal growth
- fetal development
- gestational age
- Image the following:
- Amniotic fluid index(sum of 4 quads)
- cervic should be 3-4 cm long
- lower uterine segment
- femur length
- bi-parietal diameter
- head circumference
- ambdominal circumference (most sensistive measurement for determining fetal growth)
- 4-chamber heart
- heart rate
- resistive index (umbilical cord)
- nuchal fold
- organ situs
What is the chorionic cavity?
- surrounds the amniotic cavity; the yolk sac is within the chorionic cavity
- the membrane that separates the amniotic and chorionic cavities is routinely visualized on endovaginal exams after 5 1/2 wks.
- should fuse at approximately 16-17 wks.
What is the amniotic cavity?
- cavity in which the fetus exists
- forms early in gestation
- fills with amniotic fluid to protect the fetus
- ***double bleb sign (amniotic cavity & yolk sac) is a sign of a viable pregnancy
- fetus should be near yolk sac
Where is the yolk sac located?
- in the amniotic cavity
- should be near the fetus
What is the frequency of the transvaginal transducer used in an ultrasound exam?
What are the functions of the yolk sac?
- ***Routinely the earliest intragestational sac anatomy seen
- 1) provision of nutrients to the developing embryo
- 2) hematopoiesis
- 3) development of embryonic endodorm, which forms the primitive gut
- Initially, the yolk sac is attached to the embryo via the yolk stalk, but with amniotic cavity expansion, the yolk sac, which lies between the amniotic and chorionic cavities, detaches from the yolk stalk at approximately 8 wks. of gestation.
- Typically, the yolk sac resorbs and is no longer seen sonographically by 12 wks. Persistent yolk sac does occur and may be visualized at the placental umbilical cord insertion where the amniotic and chorionic membranes are fused.
At what age will the embryonic heart start beating?
day 35 (5.14 wks.)
- TA: 6-6.5 wks.
- TV: 5-5.5 wks.
What is the double decidual sac sign?
the interface between the decidua capsularis and the echogenic, highly vascularized endometrium
- decidua basalis--the villi on the burrowing side of the conceptus
- decidua capsularis--the villi covering the rest of the developing embryo
What is the growth rate of the gestational sac?
1 mm. per day
seen as a 1-2 mm. sac with an echogenic ring having a sonolucent center by the 5th week of pregnancy
What produces progesterone in early pregnancy?
- the most common cystic structure seen in adnexa during 1st. trimester
- seen in ovary
- should be anechoic (if internal debris present, then it's referred to as a hemorrhagic cyst)
What is a physiological intestinal fetal hernia?
- the anterior abdominal wall and primitive gut are developed by 6 wks.
- because the midgut is in direct communication with the yolk sac, amniotic cavity expansion pulls the yolk sac away from the embryo, forming the yolk stalk
- as amniotic expansion occurs, the midgut elongates faster than the embryo is growing, causing the midgut to herniate into the base of the umbilical cord.
- continues to grow and rotate until 10 wks before it descends into the fetal abdomen at about the 11th wk.
- should be resorbed and not seen by the 12th wk.
What is the cystic structure seen in the fetal cranium in the 1st. trimester?
rhombencephalon (hind brain)
What is the other name for the gestational sac?
? decidua capsularis ?
How many vessels are in the umbilical cord?
- umbilical arteries (2)
- umbilical vein (1)
Which is the most accurate measurement for gestational age?
crown-rump length and gestational sac measurement in early pregnancy
where is hCG found?
- hormone secreted by the trophoblastic cells (developing placental cells)
- can be measured in the blood or urine of mother
- ***a normal gestational sac can be consistently demonstrated when the hCG level is 1800 mIU/ml (Second International Standard) or greater when using transabdominal sonography
- but sometimes can be seen by the time 500 units are present
- MUST see a GS by the time 1200 units are present
- doubles in the blood every 48 hours
- should be less than 5000 units if fetus is deceased
- main function is to convince corpus luteum to continue secreting progesterone and to prevent corpus luteum from becoming corpus albicans
What regulates the amniotic fluid level?
fetal swallowing and urinating
- ***Amniotic fluid is produced by:
- 3rd--kidneys **90%
By what gestational age do the amnion and chorion fuse?
What produces cerebrospinal fluid in the fetus?
choroid plexus within the lateral ventricles
What structure should be scanned to establish renal function in a fetus?
fetal urinary bladder
What is the normal measurement of a lateral ventricle?
**upper limit-10mm (ventriculomegaly)
What is the landmark needed to measure biparietal diameter?
- Must visualize the falx cerebri and one of the following:
- 1) Thalamus
- 2) Cavum septum pellucidi
***should, if possible, also visualize the choroid plexus in the atrium of each lateral ventricle
What is the heart rate in an embryo?
What is the heart rate in a fetus?
- 90-115 at 6wks.
- 140-160 at 9 wks.
- approximately 140 throughout the remainder of the late 1st and 2nd trimesters
- normal to be 120-160 for duration of pregnancy
What is a blighted ovum?
anembryonic pregnancy-when the mean sac diameter is 25 mm or higher, and no yolk sac or embryo is seen
- ***if yolk sac is present, but no embryo, then it's early pregnancy failure
- if yolk sac and embryo are both present, but there is no detectable heart tone, then it's embryonic demise
What is nulligesta?
?? never been pregnant?
***this is a word that appears in Italian, Spanish and French(spelled nulligeste) texts, that as far as I can tell, translates to "never conceived a child," which should mean that it's the same thing as nulligravida, but it's nowhere in our book or listed as a definition on the interwebs. =(
What are the sonographic findings of an ectopic pregnancy?
- live embryo within the adnexa is the most specific finding for ectopic gestation
- an extrauterine sac within the adnexa is the most frequent finding of ectopic pregnancy, with an thickened echogenic ring, separate from the ovary
- there is a possibility that an embryo or yolk sac may be seen
- gestational sac with a Doppler resistive index of less than 0.40
- ***The most important finding when scanning for ectopic pregnancy is to determine if there is a normal intrauterine gestation (reducing the probability of an ectopic pregnancy) or if the uterus is empty and an adnexal mass is present.
- As many as 20% of patients with ectopic pregnancy demonstrate an intrauterine saclike structure knkown as the pseudogestational sac.
- 1) pseudogestational sacs do not contain either a living embryo or yolk sac
- 2) pseudogestational sacs are centrally located within the endometrial cavity, unlike the burrowed gestational sac, which is eccentrically placed
- 3) homogeneous level echoes are commonly observed is pseudogestational sacs, unlike normal, anechoic, gestational sacs
- ***these findings are best seen with endovaginal ultrasound
- peritrophoblastic flow can be seen (low-resistance (high-diastolic) pattern with fairly high peak velocities (approx. 20 cm/sec.))
- the decidual cast of the endometrium typically demonstrates a high-resistance pattern (low-diastolic component) with low peak velocities
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