A recent meta-analysis of progesterone supplementation during synchronisation of ovulation protocols for timed artificial insemination (AI) in dairy cows was published in the April 2015 Journal of Dairy Science. The systematic review of the literature evaluated the effects of progesterone supplementation using a single intravaginal insert during timed-AI on fertility of dairy cows under various management systems. A total of 25 randomised controlled studies including 8,285 supplemented cows and 8,398 untreated controls were included in the meta-analysis. Information regarding the presence of the progesterone-producing body the corpus luteum at the initiation of the synchronisation protocol was available for 6,883 supplemented cows and 6,879 untreated controls in 21 experiments. Studies were classified based on service number, use of pre-synchronisation, and insemination of cows in oestrus during the synchronisation protocol. The results showed that progesterone supplementation: Increased the chance of pregnancy by 10 per cent Tended to reduce pregnancy loss in the first 60 days of gestation Plus, a benefit to supplemental progesterone was observed in studies during which all cows were inseminated at a fixed time with no detection of oestrus during the synchronisation protocol and in cows without a corpus luteum at the initiation of the synchronisation protocol. Service number and presynchronisation did not influence the effect of progesterone supplementation on fertility. In summary, progesterone supplementation using a single intravaginal insert during the timed AI programme increased pregnancy per artificial insemination, mostly in cows without corpus luteum, and reduced the risk of pregnancy loss in lactating dairy cows. Insemination of cows in estrus during the synchronisation protocol eliminated the benefit of supplemental progesterone on pregnancy per artificial insemination