Cervical Sonography Flashcards
Cervical canal/ cervical length
Cavity between the internal os and external os
Approximately 35mm in length
53 mm +- 12-13mm before 33 weeks gestation
(20-35 is cutoff)
Location of cervix
Extends from the inferior end of the uterus to the superior portion of the vagina
- internal os
- external os
Cervical division
Cervix divided into supra and infra vaginal segments
Blood supply of uterus
From uterine arteries
Uterine arteries become less resistant during pregnancy
Digital exam evaluation
Evaluates:
- dilation of the external os
- % effacement (cervical length shortens)
- station of the presenting part
- position of the cervix
- consistency of the cervix
- > for baby to come out, need 100% effacement (10cm dilation of the external os)
- > cervix is soft before birth
Transabdominal pros
- less invasive
- less cumbersome (less difficult to do)
- well accepted by the patient compared to transvaginal sonography
Transabdominal cons
- poor visualization
- apparent artificial lengthening when maternal bladder is full
Transabdominal empty bladder
Overcomes bias of artificial lengthening of Transabdominal measurement with full bladder
Transperineal technique
- maternal bladder empty
- internal cervical so and upper cervical os routinely visualized
- external os may be obscured by overlying bowel gas
- put probe on top of labial flap, noninvasive
Pregnancy red flag
If cervix is shorter than 3 cm
If internal os appears to have a V or U shape
Transvaginal pros
- reliable
- reproducible
- not affected by maternal obesity, cervix position, shadowing from fetus
- may identify other US risk factors better : intramniotic debris, placenta previa, vasa previa
Transvaginal scan technique
- insert probe in anterior fornix under direct real time vision
- push posterior
- pressure in, pullout a bit, then push again
- relax probe pressure until image begins to blur, then reapply pressure for best image
Voiding before TVS exam
- Brief interval (15 min) b/w voiding and TVS cervical evaluation is a associated with decreased risk for focal myometrial contractions (can impede accurate cervical length)
- delaying cervical evaluation to later gestational age (21-23 weeks) may decrease focal myometrial contraction incident
% visualization of cervix
Transabdominal : 86% w/ full bladder
Translabial : 90%
Transvaginal : 100%
Cervical scan technique
- rotate probe to see best long axis view of the canal
- find these landmarks in order
1. Fetus and amniotic fluid
2. Bladder
3. Internal os
4. Cervical canal
5. External os
Exam length of time
Less than 5 min is not enough to detect possible changes in cervical length as well as evaluating funneling
Transvaginal cervical technique
- anterior and posterior cervical width is same size
- entire canal seen well
- both internal and external os identified
- do not get fluid in measurement
Pressure techniques
✔️ transfundal pressure (on top of uterus)
✔️ suprapubic pressure (above pubic bones)
Pressure techniques purpose
- performed to establish shortest cervical length
- observe funneling at Internal Cervical Os
Transfundal Pressure
- patient in supine position
- hand placed at fundus of uterus and downward pressure applied
Suprapubic pressure
- patient in supine position
- hand placed just superior to the symphysis pubic bone and downward pressure applied
Cervical length measurements
- wait 30 sec before measuring cervix
- 1st measurement usually longer than subsequent measurements (discard it)
- measure length repeatedly until change is less than 10%
(Take multiple measurements) - record “shortest best measurement” (do not average poor measurements)
Trouble finding a good image?
- make sure the probe is inserted far enough to see the cervix
- 1st organ seen is always the maternal bladder
Trouble finding a good image?
- find lowermost edge of the empty bladder -> internal os should be directly below
- cervical axis may not lie in mid-plane of body -> move probe left and right
Uterine didelphysis
- estimated to occur in 1/3,000 women
- uterus is present as a paired organ
- will have a double cervix
- complete separation of uterus = 2 vaginas, but one is usually blocked
- ask about success rate w/ tampons (don’t work because you need 2)
- note in which uterus the pregnancy is in
Cervical length at 24 weeks
1st percentile - 13mm 21% PTD (pre term delivery)
5th percentile - 20 mm 12%
10th percentile - 26 mm 8%
25th percentile - 30 mm 5%
50th percentile - 35 mm 3.4%
Candidates for TV US
- women w/ suspected cervical incompetence
- women w/ symptoms of preterm labor in current pregnancy (uterine contractions)
- asymptomatic pregnant women w/ risk factors for preterm birth
- wine w/ suspected placenta previa or low lying placenta
- women w/ suspected cervical pregnancy
Changes of Internal Os w/ funneling
- cervical canal shortens progressively and an opening of a funnel shaped internal cervical os occurs until complete cervical effacement is achieved
- the letters T Y V U illustrate cervical changes that can be seen
Changes of internal cervical Os
T rust
Y our
V aginal
U ltrasound
Funneling - functional length
Cervical length distal to the funnel extending to external cervical os
Funneling - funnel length
Length of a line that connects the apex of the funnel to the superior edge of the base of the funnel
- from cervix (opening up) to internal os
Cervical incompetence
- affects 1% of pregnant women
- 2nd trimester painless dilation followed by fetal expulsion
Risk factors for cervical incompetence
- in utero exposure to DES
- major uterine anomalies
- history of cervical trauma
- history of recurrent spontaneous and therapeutic abortions
- history of precipitous labor or advanced dilation before labor onset
Preterm labor (PTL)
-diagnosis is made when cervical change is noted on serial digital examinations of the cervix (do at 37-38 weeks)
Preterm birth (PTB)
Any delivery that occurs before 37 completed weeks
Signs of preterm labor
- vaginal discharge
- pelvic or lower abdominal pressure
- constant low full backache
- mild abdominal cramps
- regular or frequent contractions or uterine tightening
- PROM (premature rupture of membranes, water breaks, baby MUST come out)
Major risk factors for spontaneous PTL
- multiple gestation
- previous preterm delivery
- abdominal surgery during pregnancy
- uterine anomaly
- hydramnios (too much amniotic fluid)
- history cone biopsy
- more than one 2nd trimester termination
Surgical techniques to strengthen the cervix
- shirodkar cerclage (requires anterior displacement of the bladder
- McDonalds Cerclage (purse string technique, thread through cervix, want to place it closer to internal os)
- Transabdominal cerclage
- performed prophylactically (just in case, ahead of needed time, better safe than sorry) at 12-15 weeks gestation
Other treatment to strengthen the cervix
Progesterone supplementation
- cost effective
- safe
- accepted by patients
- widely available
Abdominal cerclage
- performed b/w 11-13 weeks
- bladder flap created
- cesarean delivery is necessary (suture is left in place if future fertility is desired)
Placenta previa
- 3 recognized variations
> total
> partial
> marginal - gestational age at time of US greatly influences incidence of placenta previa
- TVS best for visualization
Placenta previa incidence
- incidence w/ TAS is reported to be 5% during 2nd trimester - at delivery 0.5%
- incidence with TVS is reported to be 1.1% - at delivery 14%
- placenta overlapping the internal cervical os by more than 10mm identified women at risk of previa at delivery
Vasa Previa
- presence of fetal blood vessels across the internal cervical os
- blood vessels near the internal cervical os may be detected with TAS/TVS
- Color/duplex Doppler can confirm fetal origin of the vessels
Placenta previa and Vasa Previa
Both require c-section
Cervical pregnancy
- incidence varies: 1/2500 to 1:12,422
- 1% of ectopic pregnancies
- c-section scars increase the risk
Lesson #1
TVS and TPS are more reproducible than images obtained by TAS full bladder
Lesson #2
After 20 weeks gestation, the cervix appears to shorten or efface slightly with increasing gestational age
- median values: 35-40 mm at 24-28 weeks
- median values: 30-35 mm after 32 weeks
Lesson #3
2 findings that are consistently associated with an increased risk of preterm birth:
1- cervical length less than the 10th (25mm) to 25th (30mm)
2- appearance of funnel at ICO that is greater than 40-50% of total cervical length
Preme
Birth less than 32 weeks
Late pre-term
32-37 weeks, still complications involved
Funneling
Only normal after 36 weeks
