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ANTE- means "before, preceding". So ante-natal classes are classes you take BEFORE your baby is born. The ANTE- prefix forms words such as: antebellum (occurring before a particular war), antecedent, antechamber, antedate, antediluvian, and anterior.
ANTI- means "opposed to, against". So anti-natal classes would be classes "against babies" ... which sounds rather alarming! The ANTI- prefix is the prefix that forms words such as: anti-aircraft, anti-inflammatory, antibiotics, antibody, anticlimactic, anticlockwise, antidote, antigen, antihero, antiperspirant, antipodean, antipathy, and antisocial.
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