2nd/3rd Trimester Cervix and Amniotic Fluid (Ch. 51 & 58)

cervical length

• CL = distance between internal os and external cervical os

• normal length (gravid uterus) is at least 3 cm

• cervical incompetence is CL less than 2.5 cm

  • short CL is marker for increased preterm birth loss

  • risk of loss inverselt proportional to CL

normal CL vs. cervical incompetence CL

• normal is at least 3 cm

• cervical incompetence is >2.5 cm

cervix: transabdominal technique

• scan through amniotic fluid/bladder to image cervix

• measure length of cervix from internal os to external os

• do not include vaginal canal in measurement

• pitfalls:

  • maternal body habitus

  • full bladder can compress LUS and mimic a long closed cervix

cervix: translabial technique

• alternative technique for assessing cervical length

• curvilinear transducer placed over labia

• less accurate than TV technique

cervix: transvaginal technique

• GOLD STANDARD method

• bladder is empty

• define cervical canal—take close-up shot (75% of screen)

• measure length from internal os to external os

  • if cervix is curved, use trace or obtain two or more linear measurements then add them together

what is cervical incompetence

• when CL is less than 2.5 cm

• cervix does not remain closed

causes and S/S of cervical incompetence

causes:

• lacerations

• uterine anomalies

• previous trauma

symptoms:

• asymptomatic

• painless dilation

• recurrent 2nd trimester loss

is this normal or cervical incompetence?

normal

is this normal or cervical incompetence?

incompetence

cervical cerclage

• tx for cervical incompetence

• cervix is stitched closed to prevent preterm birth

• cerclage removal:

  • removed around 36-37 weeks GA for planned vaginal delivery OR

  • left in place and removed at time of planned cesarean delivery (c-section)

SONO: cervical cerclage

• post-cerclage procedure; imaged transvaginally

• cerclage stitches are echogenic with posterior shadowing

• serial scans may be done to ensure cerclage remains secure and cervix is closed

what are the arrows pointing to?

cervical cerclage stitches

amniotic fluid

• fluid surrounding fetus within amnion

• plays vital role in fetal growth and allows fetus to move freely

• produced by umbilical cord, membranes, lungs, skin, kidneys, and mostly by urinary tract

  • early 1st trimester: fetal membranes

  • late 1st trimester: skin

  • 2nd/3rd trimester: kidneys (urine)

amniotic fluid function

• protection

• allows for fetal movements

• prevents adhesions

• allows symmetric growth

• maintains temperature

• acts as reservoir to fetal metabolites before excretion

amniotic fluid volume

• amount of fluid is a balance between production and consumption

• volume of AF increases until about 33 weeks

  • 25 mL per week (11w-15w)

  • 50 mL per week (15w-28w)

    • volume doubles until week 28 (declines after that)

• adequate volume is critical for lung development

• by the end of pregnancy there is sharp decline in amount of amniotic fluid

SONO: amniotic fluid

• generally echo-free

• can sometimes see tiny echogenic particles

  • blood, normal variant, vernix caseosa, meconium

• presence of a dense collection, or sludge, could indicate infection

3 amniotic fluid measurement methods

1. subjective

2. amniotic fluid index (AFI)

3. single pocket

AF measurement methods: subjective

• eyeballing

• amniotic fluid index (AFI)

• single pocket

AF measurement method: subjective

• “eyeball” assessment of amount of fluid

• no actual measurements

• better used early in gestation

• successful with more experienced sonographer

  • leads to a more quantitative assessment

which has subjectively less AF?

right image

AF measurement method: amniotic fluid index (AFI)

• most used method for quantifying amniotic fluid volume

• uterine cavity is divided into 4 equal quadrants

• largest pocket in each quadrant is measured (AP) in SAG plane and added together

  • transducer should be perpendicular to table, not curved skin surface

  • exclude fetal limbs and umbilical cord loops )can use color Doppler)

    • do NOT measure through a structure

AF measurement method: AFI example

normal AFI values

• 10 cm < AFI < 20 cm

  • should be between 10-20 cm

borderline AFI values

• 5-10 cm (low side); 20-24 cm (high side)

abnormal AFI values

• oligohydramnios (too little)

  • AFI < 5 cm

• polyhydramnios (too much)

  • AFI > 24 cm

AF measurement method: single largest pocket (MVP)

• maximum vertical pocket assessment (MVP)

• measure largest pocket in AP

• pocket should be clear of fetal parts and umbilical cord

normal single pocket (SP or MVP) values

• 2 cm < SP < 8 cm

  • should be between 2-8 cm

abnromal single pocket (SP or MVP) values

• oligohydramnios (too little)

  • SP < 2 cm

• polyhydramnios (too much)

  • SP > 8 cm

how to measure AFI for twins?

• have a slightly lower AFI than singleton pregnancies

• AFI gives overall assessment for pregnancy not search sac

• assess each sac independently

• largest vertical pocket is more accurate in polyhydramnios

• most accurate is to use dye amniocentesis method