A method for estimating whether a female is pregnant, said method comprising measuring allele presences (D) for a plurality of genetic markers of at least one chromosome, different from the X and Y chromosome, in a sample of cell-free DNA from a potentially pregnant female; each allele presence representing the presence at a genetic marker of at least one of: a reference allele of maternal or foetal origin, and an alternative allele of maternal or foetal origin; based on said measured allele presences, determining a homozygous fraction (Fho) thereof which is associated with purely homozygous genetic markers; and estimating whether the female is pregnant based on said fraction.