The Complete German Commission E Monograph for lavender lists L. angustifolia, both flowers and essential oil, as officially “approved” and with no side effects and no contraindications. This includes internal use of 1-4 drops (20-80 mg) of the essential oil as a daily dose (Blumenthal et al 1998). The Commission E Monographs are generally regarded as the most authoritative source on the safety of herbal medicines.
This article by Robert Tisserand throws light on both the use of Lavender in pregnancy and use of the term 'Lavender'. Care must be taken to avoid confusing Spike Lavender with Lavender Angustifolia when discussing the safety of use of Lavender in pregnancy.