Complex changes in von Willebrand factor-associated parameters are acquired during uncomplicated pregnancy

Abstract

Background: The coagulation protein von Willebrand Factor (VWF) is known to be elevated in pregnancy. However, the timing and nature of changes in VWF and associated parameters throughout pregnancy are not well understood.

Objectives: To better understand the changes in VWF provoked by pregnancy, we studied VWF-associated parameters in samples collected over the course of healthy pregnancies.

Methods: We measured VWF antigen (VWF:Ag), VWF propeptide (VWFpp), Factor VIII (FVIII), and ADAMTS13 activity in samples collected from 46 women during pregnancy and at non-pregnant baseline. We also characterized pregnant vs. non-pregnant VWF multimer structure in 21 pregnancies, and performed isoelectric focusing (IEF) of VWF in two pregnancies which had samples from multiple trimesters.

Results: VWF:Ag and FVIII levels were significantly increased during pregnancy. ADAMTS13 activity was unchanged. VWFpp levels increased much later in pregnancy than VWF:Ag, resulting in a progressive decrease in VWFpp:Ag ratios. FVIII:VWF ratios also decreased in pregnancy. Most pregnancies exhibited a clear loss of larger VWF multimers and altered VWF triplet structure. Further evidence of acquired VWF qualitative changes in pregnancy was found in progressive, reversible shifts in VWF IEF patterns over gestation.

Conclusions: These data support a new view of pregnancy in which VWF can acquire qualitative changes associated with advancing gestational age. Modeling supports a scenario in which both increased VWF production and doubling of the VWF half-life would account for the data observed. We propose that gestation induces a prolongation in VWF survival, which likely contributes to increased total VWF levels and altered VWF structure.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. VWF-associated parameter data in 46 pregnancies.

Values for VWF:Ag (A), FVIII (B), VWFpp (C), FVIII:VWF Ratio (D), VWFpp:Ag Ratio (E) are presented at each time point (non-pregnant baseline n = 46, 1st trimester n = 24, 2nd trimester n = 38, 3rd trimester n = 46). The median, 25th and 75th percentiles (boxed), 1 SD (bars), and individual outlying data points (black dots) are shown for each parameter at each time point. Statistically significant differences from baseline (p<0.05 by one-way ANOVA followed by all pairwise multiple comparisons) are indicated by asterisks (*). Values can be found in Table 2.

Figure 2. Changes in VWF-associated parameters from baseline in 46 pregnancies.

The changes in VWF parameters normalized to baseline are displayed longitudinally over pregnancy for time points sampled in pre-pregnancy n = 14, 1st trimester n = 24, 2nd trimester n = 38, 3rd trimester n = 44, 38wks n = 12, and postpartum n = 41. Asterisks (*) indicate p<0.05 compared to postpartum values (one-way ANOVA, all pairwise multiple comparisons). Standard deviations, means, 25th, and 75th percentiles are omitted for clarity and can be found in Table 4.

Figure 3. Loss of higher weight VWF multimers in pregnancy.

A normalized 1% multimer gel (A) and densitometry (B) of plasma VWF multimers from a single pregnancy demonstrating the most commonly observed pattern of shifted VWF multimers (arrows) in the second trimester (orange) and third trimester (red) compared to baseline VWF (blue).

Figure 4. Change in VWF triplet structure in pregnancy.

A normalized high resolution 2.6% multimer gel (A) and densitometry (B) of the same pregnancy shown in Fig 3 demonstrating the most commonly observed pattern of shifted VWF triplet structure with relative loss of height of the major band in the 3rd trimester (red) compared to baseline (blue).

Figure 5. Shift in isoelectric focus (IEF) pattern in pregnancy.

A. A normalized plasma isoelectric focusing gel from the same pregnancy as shown in Figs 3 and 4 immunoblotted for VWF. B. Densitometry tracings demonstrate the progressive shift in VWF isolectric focus in the 2nd (orange) and 3rd (red) trimesters. In the 3rd trimester, the band volume ratio of VWF in the doublet has shifted (3∶1 to nearly 2∶1) and the upper VWF band has shifted towards the lower pH focused VWF band.