Maternal thyroid function and the outcome of external cephalic version: a prospective cohort study

The incidence of breech presentation decreases with increasing gestational age from approximately 16% at 32 weeks to 3-5% of all pregnancies at term [1]. External cephalic version (ECV) is recommended by the American College of Obstetricians and Gynecologists (ACOG) and the Royal College of Obstetricians and Gynaecologists (RCOG) as the best method to reduce the number of breech presentations and breech deliveries at term [2, 3]. Also the Dutch Obstetrics & Gynaecology Society has promoted ECV but up until now, the implementation of this guideline is rather poor and a recent study has been started to evaluate possible barriers [4]. Reported success rates vary widely from 30-80% [3]. Several factors have been associated with successful ECV outcome including parity, type of breech, placenta location, amniotic fluid index, and engagement of the breech [5–7].

Recently, our research group showed that at 36 weeks gestation women with TSH levels above 2.5 mIU/l are at risk for breech presentation [8, 9]. It is stated that fetal movements and mobility are a prerequisite for spontaneous version into the cephalic position [10]. Since maternal thyroid function is related to the probability of spontaneous cephalic presentation, it may be hypothesized that TSH may, also, affect ECV outcome [8, 9].

To the best of our knowledge, this is the first prospective study to examine the relationship between maternal thyroid hormone status and ECV success.

The overall ECV success rate, indicated by the post-procedure ultrasound results, was 55% (77/141), and was significantly lower in nulliparous women (39%: 36/93) than in multiparous women (85%: 41/48; χ² = 27, df = 1, p < 0.001). In one patient prolonged post-ECV fetal bradycardia occurred which necessitated emergency cesarean section. Transient heart rate abnormalities were observed in 3 patients. Spontaneous version into cephalic position at birth after failed ECV occurred in 2 (nulliparous) women. Spontaneous version back into breech after successful ECV did not occur.

Figure 1 shows the prevalence of successful ECV according to different TSH quartiles (25^th percentile: < 0.99 mIU/l, 25^th - 50^th percentile: 0.99 - 1.48 mIU/l, 50^th - 75^th percentile: 1.49 - 2.0 mIU/l and > 75^th percentile: > 2.0 mIU/l). Clearly, the percentage of ECV success significantly decreased with increasing TSH quartile (74% to 30%, χ² = 15, df = 3, p = 0.002).

In healthy pregnant women (without elevated concentrations of TPO-Ab and with proven sufficient iodine intake) trimester specific cutoffs for elevated TSH are still lacking. A frequently used upper TSH reference level in pregnancy is ≥ 2.5 mIU/l [11]. In the current study 20 women had a TSH above this level. Of these 20 women 5 (25%) underwent successful ECV, compared to 72 successful ECVs from the sample of 121 women (60%) with TSH levels below the cutoff of 2.5 mIU/l (χ² = 8.2, df = 1, p = 0.004). The RR of women in breech with TSH above 2.5 mIU/l for failed ECV was 2.4, (95% CI = 1.1 - 5.3).

In the current study, successful ECV outcome was independently related to maternal TSH: women with unsuccessful ECV had significantly higher TSH levels.

The ECV success rate of 55% in the current study is comparable to others reported in the literature [2, 3]. While our data support earlier observations showing that factors affecting ECV success include parity, type of breech and placenta location, to our knowledge, this is the first study reporting on a possible relationship between maternal thyroid function and ECV outcome [5–7].

We recently showed in a large sample of over 1000 pregnant women from the general population that higher levels of TSH constitutes a risk factor for breech presentation [8].

How can the observed relationship between maternal TSH and ECV outcome be explained? Several mechanisms might be suggested. First, one could argue that elevated maternal TSH directly impacts uterine relaxation, thereby negatively affecting ECV outcome. Empirical evidence regarding such a direct effect is, however, scarce. TSH receptors have been shown in other non-thyroid tissues such as bone, heart and brain tissue, but so far no studies on human uterine samples have been published [12, 13]. Also, in patients with subclinical hypothyroidism (reflected by high TSH), the myocytes of the smooth muscle cells of the aorta are dysfunctional, leading to increased arterial stiffness and impaired diastolic function [14]. The mechanism behind is a reduced activity of the sarcoplasmatic reticulum CA-ATPase which controls the efficient concentration of calcium in the cytoplasm. Another study in sub-clinical hypothyroid women showed a beneficial effect of thyroxine replacement on arterial stiffness [15]. It is known from earlier studies that thyroid hormone (T3) has a direct relaxation effect on vascular smooth muscle cells [16]. It might be hypothesized that a similar relation between TSH and uterine myocytes does exist. Inappropriate relaxation of uterine smooth muscle vessels in women with high TSH might hinder ECV. Another candidate mechanism might relate to fetal motility and its ability to facilitate spontaneous version. High maternal TSH during gestation has been associated with impaired neonatal motor function [17]. One may argue that higher levels of maternal TSH could conceivably reflect low circulating fetal T4 levels. Shortage of fetal T4 compromises fetal neural maturation and, in turn, fetal motility through its effects on muscle tone and reflexes [18]. Furthermore, breech babies tend to be smaller, have lower scores on neuromotor tests, and are balanced-impaired until the age of 12-18 months [19–22]. Their impaired neuromotor development could in fact hinder ECV outcome: it might be suggested that poor fetal movements result in less expanded intra-uterine environment which is needed for successful ECV.

The current study findings did not reveal differences in FT4 level between the successful versus the unsuccessful ECV groups. This may have been due to the greater sensitivity of TSH - compared to FT4 - to detect small changes in the set point of the hypothalamus-pituitary-thyroid axis [23]. Moreover, it has been reported that pregnancy FT4 assessments are less reliable than non-pregnancy assessments [24]. Also in this study a weak correlation (r = 0.18, p = 0.34) was found between FT4 and TSH, but in the opposite direction of what would be expected: normally there is an inverse correlation between TSH and FT4.

Despite growing insight into the prognostic factors of successful ECV, no scoring system that accurately predicts success exists [25–28]. It may well be that the inclusion of maternal TSH would lead to enhanced probability-of-ECV success estimates.

A limitation of the current study relates to the fact that iodine excretion data in mothers and neonates were not assessed. This made it impossible to relate higher TSH levels to inadequate iodine intake during gestation. Also cord blood was not assessed and therefore fetal TSH and T4 levels could not be taken into account.

A major strength of the current study was its relatively large sample size and the fact that ECV was performed in one obstetric department by a small team of trained experts following the same protocol and blind to the thyroid status of the patients.

In conclusion, the current study is among the first to report a possible relationship between maternal thyroid function and ECV outcome. While the study suggests that higher levels of TSH are independently related to ECV outcome, the precise mechanism behind this finding remains to be determined.