THROMBOCYTOPENIA IN PREGNANCY
Incidence of ITP in pregnancy
The advent of routine platelet counting using automated blood cell counters highlighted the fact that mild to moderate thrombocytopenia is common in healthy women with an apparently normal pregnancy. Excluding those cases with spurious counts due to EDTA-induced in vitro platelet aggregation, the large majority of these women have 'gestational thrombocytopenia' (GT), a benign selflimiting phenomenon with no significant bleeding-risk to mother or infant. However, it may be dif?cult or impossible to distinguish GT from idiopathic (autoimmune) thrombocytopenia (ITP) in which the transmission of platelet antibodies across the placenta has the potential to cause fetal or neonatal thrombocytopenia and haemorrhage. Maternal thrombocytopenia may also be an indicator of signi?cant complications of pregnancy such as pre-eclampsia or disseminated intravascular coagulation (DIC) in addition to a range of less common acquired or congenital disorders (Table I). Thus, the assessment of individual cases of thrombocytopenia in pregnancy focuses on excluding important secondary causes and weighing the risks of bleeding in mother and infant against the hazards of diagnostic and therapeutic interventions.
Normal platelet count in pregnancy
Most pregnant women have platelet counts within the nonpregnant reference range. However, large prospective studies (level III evidence) have confirmed that platelet counts tend to fall during pregnancy and levels of 120–150×109/ l are not uncommon in the third trimester (Sill et al, 1985; Burrows & Kelton, 1988). A single-institutional study (Burrows & Kelton, 1988) of 1357 women with normal pregnancy, delivering at term, showed a mean platelet count of 225×109/ l with 95% con?dence intervals of 109–341×109/ l.
PRESENTATION AND DIAGNOSIS
Gestational thrombocytopenia (incidental thrombocytopenia of pregnancy)
The best evidence for the natural history of this phenomenon comes from a sequence of prospective studies reported by the McMaster group in Ontario (Burrows & Kelton, 1988, 1990, 1993) (Level III evidence). At term, thrombocytopenia was present in 5.4–8.3% of healthy mothers. Platelet counts were only mildly reduced in most patients (mean 135×109/ l) and were between 100 and 150×109/ l in 95% of cases. It is exceptional for the platelet count to fall below 80×109/ l but rare cases, subsequently confirmed as GT, had counts as low as 50×109/ l.
The incidence of thrombocytopenia in cord blood samples taken from the infants of women with GT and women with normal platelet counts was identical at 4%.
Hence, GT is characterized by:
♦ Mild thrombocytopenia (rarely < 80×109/ l)
♦ Occurrence in healthy women with otherwise normal blood counts
♦ Most commonly occurs in the third trimester of pregnancy
♦ Normal platelet counts before and after pregnancy
♦ No association with maternal haemorrhage
♦ No association with fetal or neonatal thrombocytopenia GT is dif?cult to distinguish from ITP when thrombocytopenia is identi?ed for the first time during pregnancy and no previous counts have been documented.
Idiopathic (autoimmune) thrombocytopenia (ITP)
Chronic ITP is most common in women of reproductive age and is therefore encountered in pregnancy. The estimated prevalence is 1–5 cases per 10,000 pregnancies (Kessler et al, 1982), i.e. around 100 times less common than GT. Patients occasionally present for the first time with severe thrombocytopenia in pregnancy and women with previously diagnosed ITP may experience an exacerbation in pregnancy, the nadir platelet count usually occurring in the third trimester (Burrows & Kelton, 1992). However, the most common presentation is the ?nding of asymptomatic thrombocytopenia on routine laboratory testing, when the distinction from GT may be difficult.
As in the non-pregnant patient, the diagnosis of ITP is largely one of exclusion as there is no con?rmatory laboratory test (Burrows & Kelton, 1992). Documentation of a low platelet count outside pregnancy is invaluable. It is important to exclude pre-eclamptic syndromes, DIC or autoimmune disorders such as SLE and the antiphospholipid syndrome, which carry signi?cant prognostic and therapeutic implications (Table II). Patients should be carefully examined for hepatosplenomegaly and lymphadenopathy or features of pre-eclampsia (hypertension, proteinuria, intrauterine growth restriction).
If there are no additional clinical features and the blood count and ?lm are otherwise normal, bone marrow examination is not recommended (Letsky & Greaves, 1996).
As with non-pregnant adults and paediatric ITP, platelet-associated IgG is of no diagnostic value. Despite recent PT, prothrombin time; APTT, activated partial thromboplastin time; DRVVT, dilute Russell's viper venom time. innovative methodologies, measurement of serum platelet autoantibodies are not clearly diagnostic of ITP in individual patients and do not predict the likelihood of neonatal thrombocytopenia (Burrows & Kelton, 1992; Letsky & Greaves, 1996; Sainio et al, 1998; Boehlen et al, 1999).
