Thromboembolism and in vitro fertilization – a systematic review
Conflict of interest
Margareta Hellgren has received research support, lecture honoraria and consultancy fees from CSL Behring, Leo Pharma, Octapharma Nordica, and Pfizer AB, Sweden. The other authors have stated explicitly that there are no conflicts of interest in connection with this article.
Abstract
Introduction
There is no accepted consensus on thromboprophylaxis in relation to in vitro fertilization (IVF). We aimed to study the frequency of thromboembolism and to assess thromboprophylaxis in relation to IVF.
Material and methods
We performed a systematic review. All study designs were accepted except single case reports. Language of included articles was restricted to English.
Results
Of 338 articles, 21 relevant articles (nine cohort studies, six case–control studies, three case series, and three reviews of case series) were identified. The antepartum risk of venous thromboembolism (VTE) after IVF is doubled (odds ratio 2.18, 95% CI 1.63–2.92), compared with the background pregnant population. This is due to a 5- to 10-fold increased risk during the first trimester in IVF pregnancies, in turn related to a very high risk of VTE after ovarian hyperstimulation syndrome (OHSS), i.e. up to a 100-fold increase, or an absolute risk of 1.7%. The interval from embryo transfer to VTE was 3–112 days and the interval from embryo transfer to arterial thromboembolism was 3–28 days. No robust study on thromboprophylaxis was found.
Conclusions
The antepartum risk of VTE after IVF is doubled, compared with the background pregnant population, and is in turn related to a very high risk of VTE after OHSS in the first trimester. We recommend that IVF patients with OHSS be prescribed low-molecular-weight heparin during the first trimester, whereas other IVF patients should be given thromboprophylaxis based on the same risk factors as other pregnant women.
Abbreviations
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- ART
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- assisted reproductive techniques
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- ATE
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- arterial thromboembolism
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- ET
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- embryo transfer
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- IVF
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- in vitro fertilization
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- LMWH
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- low-molecular-weight heparin
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- OHSS
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- ovarian hyperstimulation syndrome
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- TE
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- thromboembolism
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- VTE
-
- venous thromboembolism
Key Message
There is a very high risk of thromboembolism in in vitro fertilization complicated by ovarian hyperstimulation syndrome. Thromboprophylaxis is warranted in the first trimester when ovarian hyperstimulation syndrome occurs.
Introduction
Infertility affects 10–15% of couples trying to conceive. In 1978, Robert Edwards announced the birth of the first test tube baby 1. In vitro fertilization (IVF) has been rapidly growing as the treatment of choice all around the world. Today, approximately 5 million babies are born after IVF. Sweden contributes to this number with around 19 000 IVF treatments per year, resulting in 3000 deliveries, corresponding to 3% of all neonates 2. Postponed childbearing, new medical treatment options for serious illness and a growing demand for fertility preservation in women with malignant disease contribute to a steady increase in the demand for assisted reproductive techniques (ART). Safety aspects of treatment are central and preconception risk assessments include obstetric considerations, for example in the case of older women who wish to conceive by egg donation. Cross-border reproductive travelers seek treatment options not available or allowed in their own countries, and ART-related complications and complicated pregnancies are thus brought home.
Severe complications in IVF are rare but the increasing number of treatments will increase the absolute number of women affected. Ovarian hyperstimulation syndrome (OHSS) is the most common and serious of these complications 3, 4. In its severe forms, it is associated with an increase in the risk of thromboembolism (TE), which may be fatal. Data on the incidence of venous thromboembolism (VTE) and arterial thromboembolism (ATE) related to OHSS are limited, and are dominated by case reports in the literature.
IVF is reported to double the risk of TE in pregnancy, but the absolute risk is presumed to be low 5. The literature provides scarce data concerning the true incidence of TE and IVF and there is no generally accepted consensus on thromboprophylaxis in relation to IVF.
The aim of this systematic review was to create a basis for a Swedish guideline on thromboprophylaxis in women undergoing IVF. Primary outcome was the frequency of TE including venous and arterial complications after IVF with or without OHSS. Secondary outcomes were timing of TE onset related to IVF and reported regimes of thromboprophylaxis.
Material and methods
The Swedish Society of Obstetrics and Gynecology (SFOG) asked Hem-ARG, a working and reference group for hemostatic disorders in obstetrics and gynecology, to create an evidence-based guideline for thromboprophylaxis related to IVF. The literature was searched for relevant articles on IVF and TE. The following electronic databases were searched: MEDLINE, PubMed, Clinical Queries PubMed, and Wiley Interscience Cochrane Library.
The search lines were established with professional help from a librarian at the Karolinska University Hospital. A MEDLINE search was performed with the following MeSH-terms:(((((((Reproductive Techniques, Assisted)) OR (ivf)) OR (egg donation))) AND (((thrombo prophylaxis)) OR (“Thrombosis”[Mesh] OR thrombos* OR dvt OR vte)))) OR ((((((Reproductive Techniques, Assisted)) OR (ivf)) OR (egg donation))) AND (((thrombophilia)) OR (coagulation disorder))). The search covered the period 1 January 1966 to 31 December 2016. Additional searches were made in Clinical Queries PubMed with the search term “fertilization in vitro AND thrombosis,” as well as in the Cochrane Database of Systemic Reviews and in the Cochrane Central Register of Controlled Trials with the search terms “fertilization in vitro” and “thromboembolism.”
All study designs were accepted except single case reports. Review articles of case series were included to avoid missing studies. Cross-references were read and additional articles were found and included. The language of included papers was restricted to English.
Exclusion criteria were: animal studies, biochemical studies, articles not addressing the subject, articles without patient data, and reviews consisting of already included articles.
A first selection was made by three of the authors (R.H., E.N., M.S.), who all undertook an overview of all titles and abstracts and selected the articles identified as relevant for this systematic overview. All articles were then read, tabulated and evaluated (Table 1). Only studies with valid control group were used for calculations of frequency of TE and the risk of VTE during the first trimester. Meta-analysis was done with REVIEW MANAGER 5.3 (The Nordic Cochrane Centre, Copenhagen, Denmark).
Year | Ref | Author | Study design | Aim | Population (n) | OHSS (n) | Cycles (n) | Control population (n) | Incidence TE pc/ppreg (n) | Thromboembolic events (n) |
---|---|---|---|---|---|---|---|---|---|---|
1993 | 22 | Delvigne A | Case-con | Prediction of OHSS | 128 | 128/118 sev | na | 256 | na | VTE 1/128 (1 UBVTE) |
1995 | 8 | Kodama H | Case ser | Hemostasis in OHSS | 23 | 23 | 1316 | None | 0.08%* | 1 UBATE |
1998 | 9 | Abramov Y | Case-con | Severe OHSS | 163 | 163 | 163 | None | 2.5%* | 4 VTE (4 PE) |
1998 | 6 | Serour GI | Case-con | IVF | 2924 | 222 | 3500 | None | 0.17%* | 6 TE (4 VTE + 2 ATE) |
1998 | 19 | Aboulghar MA | Case- con | OHSS with CVT | 2 | 2 | na | None | na | 2 ATE |
2002 | 15 | Dulitzky M | Cohort | Thrombophilia in OHSS | 20 | 20 | na | 41 | na | 3 VTE (2 PE and 1UBVTE) 17/20 thorombophilia |
2004 | 10 | Grandone E | Case-con | Prevalence TE | 305 | na | 747 | None | 0.5%* | 3 TE (1 ATE + 2 [ATE + VTE]) |
2006 | 7 | Yinon Y | Cohort | Thromboprophylaxis | 24 | na | 74 | None | 0 | 0 VTE 0 ATE |
2006 | 12 | Chan WS | R-case ser | UBVTE in IVF | 2 | na | 2500 | None | 0.08%* | 2 VTE + 32 cases reviewed |
2007 | 20 | Girolami A | R-case ser | Risk of ATE | 34 | na | na | None | na | 34 ATE (19 UBATE) |
2008 | 21 | Jacobsen AF | Case-con | Assessment of risk factors for VTE |
ap = 268 pp = 291 |
na | na | 1229 | na |
ap 20/268 vs. 23/1229 OR = 4.3 pp 8/291 vs. 23/1229 OR = 2.6 |
2009 | 24 | Salomon O | Case ser | Explain UBVTE | 5 | na | na | None | na | 5 UBVTE |
2009 | 17 | Chan WS | R-case ser | Risk of ATE and VTE | 96 | 79 | na | None | na | 35 ATE (90% OHSS), 61 VTE (78% OHSS) |
2011 | 16 | Ricci G | Cohort | Thrombophilia in OHSS | 480 | na | 1105 | 490 | 0* | 0 VTE 0 ATE |
2011 | 26 | Gbaguidi X | Cohort | Risk for UBVTE | 1948 | na | na | None | na | 5 OHSS (17.2%) UBVTE |
2012 | 5 | Rova K | Cohort | VTE in IVF (giving birth) | 19 194 IVF | 1291 | na |
935 338 0.017% |
0.17% 1st trim 1.7% OHSS 1st trim** | VTE 32/19194 IVF OR = 9.8 VTE/1113 OHSS OR = 100 |
2012 | 18 | Hansen AT | Cohort | VTE in IVF(not pregnant) | 30 884 | na | 75 141 | Nonea | 0.009% VTE 0.003% ATE* | 7 VTE, 2 ATE |
2012 | 23 | Fleming T | Case ser | UBVTE with OHSS | 2 | na | na | None | na | 2 VTE OHSS |
2013 | 13 | Henriksson P | Cohort | VTE (giving birth) | 23 498 IVF | na | na | 116 960 |
0.42% 0.25% control** |
99 VTE, control 291 VTE 1st trim 0.15%, 0.03% control |
2014 | 14 | Hansen AT | Cohort | VTE in IVF (giving birth) | 16 191 women 18 787 preg | 549 | na | Nonea | 0.29%** | 36 VTE, ap 0.29% HR = 3.0, 12 VTE pp 0.28% HR = 1.2 1st trim HR 5,9, 2nd trim HR 2.4 |
2015 | 11 | Villani M | Cohort | Incidence of VTE in IVF (giving birth) | 234 | 10 | 684 |
3339 0.18% |
0.85% VTE** 0% OHSS |
2 VTE case, 6 VTE control |
- ap, antepartum; ATE, arterial thromboembolism; Case-con, case–control; Case ser, case series; HR, hazard ratio; na, not applicable; OHSS, ovarian hyperstimulation syndrome; OR, Odds ratio; pc, per cycle (marked *); PE, pulmonary embolism; pp, postpartum; ppreg, per pregancy (marked **); preg, pregnancies; R-case ser, review case series; ref, reference number; sev, severe; trim, trimester; UBATE, upper body ATE; UBVTE, upper body VTE; VTE, venous thromboembolism.
- a Assumption of prevalence in background population of the authors.
Results
A total of 338 articles were identified. After the first selection, 60 articles (17 reviews and 43 others) were considered relevant for further exploration, and were carefully studied by the group. A total of 19 articles from database searches were eligible. Two articles, relevant for the subject, were found in cross-references after the initial search and a total of 21 articles were thus finally included in this systematic review (Figure 1) 6, 7. We considered the overlap between reviews of case-series when results were presented, in order not to present data more than once. Details of included studies are presented in Table 1.

Risk of TE in relation to IVF
The frequency of TE during pregnancy in patients after IVF, with or without OHSS varies between 0.8 and 25/1000, compared with 0.17–2.5/1000 in the background pregnant population (Table 1) 5, 6, 8-14. The study by Dulitzky et al. 15 and Ricci et al. 16 included only women with thrombophilia and were therefore not included in these figures. In studies assessing the risk of antepartum VTE, the reported risk was approximately doubled, odds ratio (OR) 2.2 (95% CI 1.6–2.9) (Figure 2) 5, 11, 13, 14, 17 and the reported risk of first-trimester VTE was increased 5- to 10-fold (OR 6.4, 95% CI 4.0–10.1) (Figure 3) 5, 13, 14. The risk of VTE after IVF failing to lead to conception was not increased compared with a reference population 18. ATE was rare and a high propensity for ATE in relation to OHSS was reported (Table 1) 6, 10, 12, 17, 19, 20. Data regarding postpartum VTE were diverse 13, 14, 21.


Risk of TE in relation to IVF complicated by OHSS
Ten articles concerning TE in connection with OHSS presented TE results as either the primary or secondary outcome (Tables 1 and 2) 5, 6, 8, 9, 11, 14, 15, 17, 19, 22. In the study by Rova et al., the subgroup of women conceiving after fresh IVF and hospitalized due to OHSS had a 1.7% risk of VTE in the first trimester, compared with a 0.017% risk in the background non-IVF population, a 100-fold increase 5. The corresponding result published by Hansen et al. was a 14-fold increased risk, after exclusion of high-risk cases 14. Hansen et al. also reported that women with polycystic ovary syndrome (PCOS), the major risk factor for OHSS, had a similarly increased risk 14.
Timing of TE in relation to IVF
The reported interval from embryo transfer (ET) to VTE was 3–112 days 5, 11, 12, 17, 23, 24. In the review by Chan 12, this interval from ET to VTE was shorter (mean 18 days, range 3–49 days) in the group contracting OHSS than in the group without OHSS (mean 57 days, range 14–105 days) (Table 2). The reported interval from ET to ATE was between 3 and 28 days 8, 17, 19, 20.
Thromboprophylaxis
Thromboprophylaxis was reported on in three studies, but only one study including 24 women had this as the primary aim (Table 3) 7, 11, 23. The two case series by Chan 12, 17 are not included since the treatments varies to a large extent within the report. Low-molecular-weight heparin (LMWH) with or without aspirin was administered in all reported studies, but mostly the type and dose were not stated. Despite administered prophylaxis, the number of TE cases was higher than expected in a normal pregnant population. There were no robust investigations regarding bleeding complications, efficacy or osteoporosis in relation to thromboprophylaxis and IVF. Allergic reactions were reported in about 2% of patients 25.
Year | Author | Study design | Prophylaxis (n) | VTE (n) | ATE (n) | TE (n) | Type of prophylaxis | Start-duration |
---|---|---|---|---|---|---|---|---|
2006 | Yinon Y | Cohort | 24 | 0 | 0 | 0 TE | LMWHa (n = 19) LMWHa + ASA (n = 5) | OI – 6–12 weeks pp |
2012 | Fleming T | Case ser | 2 | 2 | 0 | 2 UBVTE | LMWH | (1) 8 days after ET, (2) Before OI |
2015 | Villani M | Case-con | 23 (3 OHSS) | 0 | 0 | 0 TE | LMWH or LMWH + ASAb | nac |
- case-con, case–control; case ser, case series; LMWH, low-molecular-weight heparin; na, not appliccable; OHSS, ovarian hyperstimulation syndrome; OI, ovarian induction; pp, postpartum; TE, thrombotic event; UBVTE, upper body VTE.
- a LMWH 0.6–1 mg/kg.
- b LMWH + ASA doses not specified.
- c Unknown start of thromboprophylaxis.
Discussion
We conclude that the antepartum risk of VTE after IVF is approximately doubled, mainly due to a 5- to 10-fold increased risk during the first trimester, in turn primarily due to a very high risk in the subgroup complicated by OHSS 5, 14. Thrombosis connected to IVF has been shown to have a propensity to occur in the upper extremities 8, 12, 15, 20, 22, 24, 26. A suggested explanation for the increased risk of upper-extremity TE is the result of inflammatory peritoneal fluid draining through the thoracic ducts 27. ATE was reported more often in subgroups of patients with OHSS, but as most reports are case studies, the degree of the upper-body propensity and the strength of the relation between ATE and OHSS are as yet unknown. Thus, large studies are needed.
Biological explanations for the increased incidence of TE associated with IVF may comprise normal physiological changes in pregnancy, resulting in a hypercoagulable state with increased risk of VTE 28-30. Increased estrogen levels may impact the state of hypercoagulability 31. Studies during ovarian stimulation, before and after ovulation induction, show activated coagulation 32-34. Patients developing OHSS were found to have increased levels of hemostatic markers compared with those who did not contract OHSS, as well as in comparison with a control group of healthy women 35.
Most studies are small and many of them have not reported confounders in the control group. There is a risk of bias concerning age of women, body mass index, and obstetric complications. Another weakness of this review is the risk for selection bias due to the inclusion of case series 8, 12, 17, 20, 23, 24. However, we only included larger studies in meta-analysis in order to compare robust data. Furthermore, case series may increase the risk for publication bias 12, 17, 20, 23, 24, 36. Regarding postpartum VTE, a Norwegian hospital-based, case–control study found a fourfold increased risk for VTE antepartum in pregnancies after IVF (OR 3.8, 95% CI 1.8–8), but the postpartum TE risk was not increased 21. The results of two Swedish studies 5, 13 were contradictory; however, the one reporting increased postpartum risk had included cases long before the modern thromboprophylaxis algorithm was implemented in Sweden 37.
The studies, albeit mostly small, are fairly consistent regarding timing of VTE onset after IVF, ranging from 3 to 112 days after ET 5, 12, 17, 23, 24. Thus, the risk period was longer than has previously been perceived and the increased risk persisted throughout the whole first trimester. The small studies did not allow comparison of the time intervals ET to ATE and ET to VTE.
The proper dosage and the duration of LMWH administration in relation to IVF are uncertain as they cannot be determined from the literature. The Royal College of Obstetricians and Gynecologists states that LMWH should be given on an individualized basis in cases of OHSS 38. Thromboprophylaxis with LMWH during pregnancy is related to a relative risk-reduction of up to 88% at appropriate doses of LMWH 25, 39-41. However, LMHW in pregnancy has been reported to be related to a low (2%) but increased risk of bleeding, postpartum hemorrhage and hematomas 25, 39, 42. The occurrence of osteoporosis in relation to LMWH thromboprophylaxis seems to be substantially lower than with unfractionated heparin 43-45.
In Sweden, thromboprophylaxis is recommended to all pregnant women at an estimated risk of VTE at least similar to the antepartum risk of women with one prior VTE (i.e. ≈5% absolute risk during all three trimesters, or 1.7% per trimester) 39. The risk of VTE related to OHSS in the first trimester is 1.7% 5 Therefore, in the absence of additional risk factors for VTE, LMWH is recommended to be administered to OHSS patients during the whole first trimester, but not thereafter 5, 39, 46.
In conclusion, the antepartum risk of VTE in pregnancies after IVF is doubled that in the background pregnant population, mainly due to a 5- to 10-fold increased risk during the first trimester. This risk is related to a very high risk of VTE during the entire first trimester after OHSS. The recommendation from our group of authors and clinical experts (an expert opinion) is that IVF patients with OHSS should be prescribed LMWH during the first trimester. Other IVF patients should be given thromboprophylaxis based on the same risk factors as other pregnant women.
Acknowledgments
This systematic review by the working and reference group for hemostatic disorders in obstetrics and gynecology (Hem-ARG) was commissioned by the Swedish Society for Obstetrics and Gynecology. The authors would like to thank Dr. Joy Ellis for language review.
Funding
No special funding.