VBAC Scare Tactics (11): Your Uterus is Too Thin to Attempt a VBAC
Many women who want to have a vaginal birth after cesarean (or VBAC, pronounced “vee-back”) in the U.S. and elsewhere have faced some sort of opposition from their care providers when they have expressed their desire to VBAC. Oftentimes, this opposition comes in the form of ” VBAC scare tactics.”
The (outrageous) statements are often misleading, exaggerated efforts by OBs (and yes, even midwives) to discourage women from VBAC and to encourage them to “choose” a repeat cesarean. (Of course, it’s not really a choice if your provider won’t even “let” you VBAC, is it?)
If you find yourself up against a barrage of scare tactics–as I once did–it can be exceedingly difficult to stake your claim and argue against the doctor (again, or midwife) who may or may not have your and your baby’s health prioritized higher than medico-legal concerns and who may or may not be insinuating that VBACs are synonymous with driving your child in a car without a car seat.
If you do find yourself facing such scare tactics, and if you do want to have a VBAC, there are some questions that your care provider should be able to answer when s/he hurls those scary and/or outrageous comments and standards your way. And if s/he refuses to or even cannot answer these questions, then you might want to consider finding an alternative care provider–one who is making medical decisions based on research, evidence, and even respect for your patient autonomy and not on fear, willful ignorance, or even convenience.
I encourage all mothers who read this post (and others in my “VBAC Scare Tactics Series”) to take the information contained herein as a springboard from which they can 1) continue their research on VBAC, 2) maintain a communicative relationship with their care providers, and 3) find a care provider who best supports the mother’s interests and plans for the birth of her child. These posts are NOT a replacement for discussions with care providers, nor are they intended to be medical advice.
Scare tactic #11: Your lower uterine segment (LUS) is measuring way too thin for a VBAC. You’ll need to schedule a repeat cesarean so that your uterus doesn’t rupture during labor.
Questions to ask your care provider:
- What does the research say about the correlation between “thin” LUS measurements and the rate of uterine rupture?
- Are these measurements used specifically to predict uterine rupture?
- Can this measurement tell you exactly what my risk of uterine rupture is during a VBAC attempt?
- What do the major OB/GYN organizations (such as ACOG) have to say about these measurements in regard to VBAC?
A more nuanced analysis:
In recent years, some maternity care providers have begun requiring late-term ultrasounds to measure the “scar thickness” or “lower uterine segment/LUS” of their patients who are planning a VBAC.
The reasons for performing these ultrasounds is most likely based on current research examining the relationship between LUS thickness and rates of uterine scar abnormalities (such as uterine rupture or uterine scar dehiscence) in women with prior cesarean sections. Many of these studies have found slightly higher incidences of uterine rupture or scar dehiscence in groups of women whose LUS measured relatively “thin” when compared with other women in the research group. Moreover, they have often concluded that a relatively “thick” LUS can help to predict a safe VBAC attempt—that is, one with a very low likelihood of uterine rupture.
When applying this research in their practices, OB/GYNs or midwives will often risk out women from attempting a VBAC if their LUS measures below a particular, pre-determined “cut-off point.” Other care providers might even perform these sonograms at their patients’ request so as to alleviate any fears that their patients have about uterine rupture.
But as promising as these ultrasound measurements may seem—that is, promising in their ability to predict the likelihood of uterine rupture—their practical use is currently controversial within the obstetrical community. This is partly because maternity care providers and researchers have yet to determine how best to apply the research on LUS measurements in the practical setting.
What is the “lower uterine segment”?
The lower uterine segment is the part of the uterus closest to the cervix—that is, the part of the uterus where one would find a low transverse cesarean scar.
It should be noted that even in a uterus without a cesarean scar, the LUS will still be the thinnest part of the uterus at the end of a pregnancy. In fact, it becomes increasingly thinner throughout a pregnancy, as the fetus grows and the uterus expands.[i] What’s more, research has shown that lower uterine segments with cesarean scars are, on the whole, thinner than lower uterine segments without cesarean scars.[ii] Some studies have also shown that multiparous women (or women who have previously given birth) generally have a thinner LUS than nulliparous women (or women who have never given birth).[iii] [iv] Some research has even found that women who experienced some labor before their prior cesareans have a thinner LUS than those women who did not experience any labor before their prior cesarean.[v] [vi]
Ultimately, the fact that the LUS is relatively thinner than other parts of the uterus at the end of pregnancy isn’t in itself problematic. This is actually quite normal. The concern with these LUS measurements, however, is that an LUS measuring below a specified cut-off point or thickness standard may be associated with a higher risk of uterine rupture in women attempting VBAC.
Why is this measurement controversial when it comes to VBAC management?
To date, there is no consensus regarding the optimal or “safe” cut-off point or thickness standard for lower uterine segment measurements in women with prior cesarean sections. In recent medical literature on the topic, these standards range anywhere from 1.5 mm to 3.5 mm. (Some of these cut-off points are included in the table in the subsequent section.)
What’s more, there is no consensus regarding the optimal method for performing this measurement. Some studies use transabdominal sonograms (or ultrasounds where the transducer runs over a woman’s abdomen), while others use transvaginal sonograms (or ones where the transducer is inserted into the vagina). Some even use both. Still others measure the full thickness of the LUS, while others measure the myometrial layer of the uterus. And still others suggest that it’s not the relative thinness or thickness but rather the level of discrepancy between the full thickness and the myometrial thickness that is associated with a higher rate of uterine rupture.[vii]
Finally, the studies themselves examine fairly small populations of women. Specifically, most sample sizes in these studies are under 200, and none exceed 1000. In order to make any definitive standards for OB/GYNs or midwives to use in their practices, studies such as these would need not only to examine the LUS measurements of many thousands of women but also to determine an agreed upon method of sonographic measurement. Some researchers are hopeful that future studies on this topic will take on these challenges in order to develop more meaningful conclusions (and practice policies) on this matter.[viii] Worth noting, however, is that neither the American College of Obstetrics and Gynecology nor the recent NIH Consensus Statement on VBAC included any specific guidelines or recommendations regarding LUS measurements and VBAC. One can infer, then, that they have deemed the current research inadequate for offering any insight regarding such recommendations.
What does the research demonstrate?
Generally, the research on LUS measurements and VBAC management has found that a relatively thick LUS can help to predict a “safe” trial of labor after previous cesarean section. While a relatively thin LUS cannot predict a uterine rupture, thin measurements are often associated with higher rates of uterine rupture or scar dehiscence (again, among relatively small populations of women).
Listed below are a number of studies on LUS measurement and its relationship to uterine rupture and/or VBAC safety in women with a prior cesarean scar. Included in this table are the number of women involved in the studies, the methods of LUS measurement that the studies used, and the cut-off point or other LUS thickness standards that the studies offered.
|Study||Number of Women in Study||Measurement Specifics||Cut-off Point|
|Kushtagi et al. 2010[ix]||174 (106 with previous cesarean section)||Transabdominal ultrasound; also performed manual measurements using Vernier calipers only in women undergoing elective repeat cesarean delivery; measurement taken within a week of delivery;||3 mm (Determined to be “suggestive of stronger LUS but…not a reliable safeguard for trial of labor.”)|
|Bujold et al. 2009[x]||236||Measured full thickness and myometrial thickness only; measurement taken between 35 and 38 weeks gestation;||2.3 mm(Anything less was considered to be “associated with a higher risk of uterine rupture.”)|
|Cheung 2005[xi]||102 (with one or more previous section section[s])||Measurement taken between 36 and 38 weeks gestation; defined LUS thickness “as the shortest distance between the urinary bladder wall-myometrium interface and the myometrium/chorioamniotic membrane-amniotic fluid interface”; most used only transabdominal ultrasound, though some used both transabdominal and transvaginal ultrasound;||1.5 mm (This LUS thickness had a relatively high “negative predictive value in predicting a paper-thin or dehisced LUS.”)|
|Sen et al. 2004[xii]||121 (71 with previous cesarean section)||Used both transabdominal and transvaginal ultrasound;||2.5 mm (This thickness was described as a “critical cutoff value for safe lower segment thickness”)|
|Asakura et al. 2000[xiii]||186||Used transvaginal ultrasound;||1.6 mm (When the LUS measure more than 1.6 mm, “the possibility of dehiscence during subsequent trials of labor is very small.”)|
|Rozenberg et al. 1996[xiv]||642||Measurement taken between 36 and 38 weeks gestation;||3.5 mm (Concluded that “the risk of a defective scar is directly related to the degree of thinning of the lower uterine segment at around 37 weeks of pregnancy.”)|
What does the research not demonstrate?
It is exceedingly important to note that most of these studies do not purport to be able to predict who will experience a uterine rupture. In other words, the LUS measurement is not a surefire tool for predicting uterine rupture.
In fact, none of these studies report having a high positive predictive value when it comes to uterine rupture or dehiscence. On the contrary, most do report having a high negative predictive value when it comes to uterine rupture or dehiscence. In other words (and to reiterate), current research has not yet shown that LUS measurements can accurately predict uterine rupture. They are, however, fairly accurate at predicting who can “safely” have a VBAC—that is, who can attempt a VBAC without experiencing uterine rupture or scar dehiscence.
What can I do if my care provider recommends an LUS measurement?
As with any recommended procedure, it is always best to discuss your questions and concerns about a sonographic LUS measurement with your OB/GYN or midwife well before the ultrasound occurs. Furthermore, you do have the right to refuse this ultrasound.
Thus, in addition to the aforementioned questions, it is also important to ask:
- What “cut-off” point is your OB/GYN or midwife using?
- What study (or studies) did your OB/GYN or midwife use to determine this cut-off point?
- What was the method of LUS measurement used in this study (or studies)? Is your OB/GYN or midwife using this very same method of measurement?
- What are your options if your LUS is measuring below this cut-off point?
You’ll want to know ahead of time what cut-off point or thickness standard your care provider is using before the sonogram is performed. Similarly, it might be helpful for you if your care provider cited the source(s) that s/he used to determine this standard. This will allow you to do some research on your own (if you’re interested), and it should help you to formulate some specific questions about the measurement itself.
Remember, there are fairly wide variations among the cut-off points or thickness standards in the research on this topic, and it is entirely within your right to know why your care provider chose one cut-off point over another. What’s more, there are fairly wide variations in measurement methods, and you want to make sure that your care provider replicates the method that was used in the study or studies from which s/he determine his or her cut-off point or thickness standard.
Moreover, it is important to know what your options are if your LUS measures below your care provider’s cut-off point or thickness standard. Would this automatically risk you out of a VBAC attempt under this care provider? Would your care provider use this information to caution you against VBAC, while still “allowing” you to have a trial of labor? Or would your care provider simply proceed with caution during labor, perhaps ruling out pitocin induction, augmentation, or any other intervention that could increase your risk of uterine rupture?
Finally, it might be helpful to put the information gleaned from an LUS measurement into perspective. For while an LUS measurement that falls below your care provider’s cut-off point might suggest that you have an increased likelihood of uterine rupture or scar dehiscence, this does not mean that you absolutely will experience either of these outcomes during a VBAC attempt. Furthermore, while a measurement that falls at or above your care provider’s cut-off point might suggest that you have an exceedingly low likelihood of experiencing uterine rupture or scar dehiscence, this does not mean that you absolutely won’t experience either of these outcomes during a VBAC attempt.
For while the aforementioned preliminary studies do suggest that LUS measurements can provide useful information regarding the risk of uterine rupture and scar dehiscence in a VBAC labor, none of them suggest that these measurements are perfect predictors of any VBAC outcomes. Thus, your care provider should not treat your LUS measurement as such.
[i]Gotoh H, Masuzaki H, Yoshida A, Yoshimura A, Miyamura T, Ishimaru T. “Predicting incomplete uterine rupture with ultrasonography during the late second trimester in women with prior cesarean.” Obstet. Gynecol. 1995 Apr; 95(4): 596-600.
[ii] Kushtagi, P and Garepelli, S. “Sonographic assessment of lower uterine segment at term in women with previous cesarean delivery.” Arch Gynecol Obstet. 2010 Feb 10.
[iii] Cheung V, Constantinescu O, Ahluwalia B. “Sonograpic evaluation of the lower uterine segment in patients wit previous cesarean delivery.” J Ultrasound Med. 2004 Nov; 23(11): 1441-1447.
[iv] Cheung, Vincent YT. “Sonographic Measurement of the Lower Uterine Segment Thickness: Is it Truly Predictive of Uterine Rupture?” J Obstet Gynaecol Can. 2008 Feb; 148-151.
[v] Bujold E., Jastrow N. et al. “Prediction of Complete Uterine Rupture by Sonographic Evaluation of the Lower Uterine Segment.” Am J Obstet Gynecol. 2009 Sep;201(3):320.e1-6.
[vi] Jatrow N, Gauthier RJ, et al. “Impact of labor at prior cesarean on lower uterine segment thickness in subsequent pregnancy.” Am J Obstet Gynecol. 2009 Dec 28.
[vii] Bergeron ME et al. “Sonography of lower uterine segment thickness and prediction of uterine rupture.” Obstet Gynecol. 2009 Feb;113(2 Pt 2):520-2.
[viii] Cheung, Vincent YT. “Sonographic Measurement of the Lower Uterine Segment Thickness: Is it Truly Predictive of Uterine Rupture?” J Obstet Gynaecol Can. 2008 Feb; 148-151.
[ix] Kushtagi P and Garepelli S. “Sonographic assessment of lower uterine segment at term in women with previous cesarean delivery.” Arch Gynecol Obstet. 2010 Feb 10.
[x] Bujold, E. et al. “Prediction of Complete Uterine Rupture by Sonographic Evaluation of the Lower Uterine Segment.” Am J Obstet Gynecol. 2009 Sep;201(3):320.e1-6.
[xi] Cheung, VY. “Sonographic measurement of the lower uterine segment thickness in women with previous caesarean section.” J Obstet Gynaecol Can. 2005 Jul;27(7):674-81.
[xii] Sen S., Malik S., and Salhan, S. “Ultrasonographic evaluation of lower uterine segment thickness in patients of previous cesarean section.” Int J Gynaecol Obstet. 2004 Dec;87(3):215-9.
[xiii] Asakura, Hirobumi et al. “Prediction of uterine dehiscence by measuring lower uterine segment thickness prior to the onset of labor.” Journal of Nipon Medical School. 2000: 67(5) 352-356.
[xiv] Rozenberg P., Goffinet F., Phillipe HJ, Nisand I. Lancet. 1996 Feb 3;347(8997):281-4.