These are the cautions as published in the BNF - British National Formulary.
This bit is about Aspirin in general - not pregnancy specific.
Antiplatelet drugs decrease platelet aggregation and may inhibit thrombus formation in the arterial circulation, where anticoagulants have little effect.
A single dose of aspirin 300 mg is given as soon as possible after an ischaemic event, (eg a heart attack) preferably dispersed in water or chewed. The initial dose is followed by long-term treatment of aspirin 75 mg daily in order to prevent further cardiovascular disease events.
Long-term use of aspirin, in a dose of 75 mg daily, is also of benefit for all patients with established cardiovascular disease, for patients with a 10-year cardiovascular disease risk(1) of 20% or more and aged over 50 years, for patients with diabetes aged over 50 years or who have had diabetes for more than 10 years, and for patients with diabetes who are receiving antihypertensive treatment. Unduly high blood pressure must be controlled before aspirin is given.
Aspirin in a dose of 75 mg daily is also given following coronary bypass surgery. For details on the use of aspirin in atrial fibrillation see section 2.3.1, for stable angina see section 2.6 and for intermittent claudication see section 2.6.4.
Use during pregnancy
There is no known contraindication to regular daily low-dose aspirin use (50–100 mg/day) either at the time of conception, or later in pregnancy (Kozer et al., 2003). While salicylate will reach the fetus (Garrrettson et al., 1975), the amount of aspirin in fetal blood will be less than a tenth of what it is in the mother (Wolff et al., 1977). Neither is there any clear evidence that occasional full-dose use carries much risk either, and teratogenicity does not seem to be an issue (see below). However there are two studies showing that such use around the time of conception may cause a measurable increase in the risk of miscarriage (Li et al., 2003; Nielsen et al. 2004).
Conflicting reports have appeared with regard to regular full-dose use. While teratogenicity does not seem to be a problem when normal therapeutic doses are used, but there has been concern that treatment could increase the risk of maternal or fetal haemorrhage. Exposure to aspirin does not commonly cause premature duct closure or later persistent pulmonary hypertension as was once feared (de Swiet and Fryers, 1990). It is, however, clear that regular full dose usage (> 3.2 g/day) in late pregnancy can delay the onset of labour (Lewis and Schulman, 1973) and may also, if taken during labour, slow the progress of labour itself (Waltman et al., 1973).
Most of the reports suggesting that aspirin use in pregnancy could be hazardous related to pregnancies where there was regular, unregulated, addictive high-dose use.
Heparins - aspirin enhances anticoagulant effect of heparins
Ibuprofen - antiplatelet effect of aspirin possibly reduced by ibuprofen
"Impaired platelet function and risk of haemorrhage; delayed onset and increased duration of labour with increased blood loss; avoid analgesic doses if possible in last few weeks (low doses probably not harmful); with high doses, closure of fetal ductus arteriosus in utero and possibly persistent pulmonary hypertension of newborn; kernicterus in jaundiced neonates."
This is why aspirin is recommended for previous experience prior to 35 weeks as this is when it needs to be stopped in order for the blood chemistry to normalise before delivery. That said, aspirin is a relatively easy drug to reverse - a shot of vitimin K, should rapid normalization be required.
Here is what the aspirin foundation say about low dose aspirin & pre-eclampsia...
"The main causes of death within the womb and in the first week of life are foetal growth retardation, in which the foetus develops too slowly; and pre-eclamptic toxaemia, in which the mother develops high blood pressure and kidney damage, leading to convulsions and death if it is not controlled and the pregnancy is not terminated.
Pre-eclampsia and growth retardation have a common origin in the spiral arteries of the placenta through which the foetus receives its oxygen and nutrition. As in the coronary arteries in heart disease, the spiral arteries become narrowed by clots, greatly reducing the flow of blood to the foetus. The rationale for aspirin in such cases is exactly the same as for coronary artery disease.
In 1990 Professor H C S Wallenberg, of Rotterdam, reviewed the results of four trials in 122 women given aspirin for pre-eclampsia. It significantly lowered their high blood pressure, reduced their kidney damage, reduced their need for Caesarean section, and reduced the numbers of births before 37 weeks.
His results led to CLASP - the Collaborative Low Dose aspirin Study in Pregnancy - which compared aspirin 60mg per day with placebo for the prevention and early treatment of pre-eclampsia.
CLASP involved 9,364 women taking this dose of aspirin or placebo from the 12th-13th weeks of pregnancy onwards, 74% of whom were at increased risk of pre-eclampsia and 12% of whom had early signs of pre-eclampsia. Twelve per cent were included because they were at high risk of foetal growth retardation, and 7% already had signs of such retardation.
Although the aspirin treatment was linked in each group with better results than those with placebo, the results were slightly disappointing. aspirin gave a reduction in the odds of occurrence of pre-eclampsia of 13% overall, although it was greater if the treatment was started before the 20th gestational week. As for foetal growth retardation, aspirin treatment reduced it by 20%: there was an 11% reduction in its development when it was used to prevent it. None of these differences was statistically significant.
What was statistically different was a 12% reduction in premature delivery. There was also less pre-eclampsia early in pregnancy: when it did develop on aspirin treatment it developed later, which was safer for both mother and baby. The perinatal mortality (infants dying in the first week of life) in patients given aspirin for pre-eclampsia with and without foetal growth retardation was 5.3%, half that of those given placebo (10.6%). However, aspirin proved to be of no value once pre-eclampsia or foetal growth retardation was established.
There were no differences between the two treatment groups in their side effects: fears that aspirin might cause excess bleeding in pregnancy were dispelled by the trial: the proportion of women with abnormal prenatal, perinatal and postnatal bleeding was the same in the two groups.
Professor Wallenberg explained the relatively low benefits on the fact that the women chosen for the trial were at relatively low risk of developing pre-eclampsia or growth retardation. The doctors doing the trials were so convinced beforehand that the drug worked that they would not put high risk patients into the placebo-controlled trial!
He now advises that pregnant women be told that aspirin has a moderate effect in preventing toxaemia in women at low risk and a marked effect in women at high risk. The latter includes women who have had pre-eclampsia in a previous pregnancy, particularly before the 12th week. They also include women with kidney disease, high blood pressure, systemic lupus erythematosus, or insulin-dependent diabetes, or who have had a sister or mother with pre-eclampsia."
In 2007, an old colleague of mine Prof Lelia Duley (along with her colleague in Australia) did a huge meta-analysis of 31 previous trials & studies (Including the CLASP trial) & concluded that with a previous history of PE, aspirin is still indicated as it reduced the incidence of recurrance by 10% and had better 'neonatal outcomes'. (The study is published in the online version of the Lancet.)
I hope this helps. Sorry for the long answers but I thought it would be easier to answer it all in full & for those who will also ask the question later along the line!
Please just give me a nudge if Ive missed your point or you would like something clarified.
Every Woman is entitled to understand what happened in her pregnancy when pre eclampsia strikes. I hope to be able to support that process.