I was talking to a client about OPU and what frequency should oocyte aspiration occur. It’s not the first time I have had a conversation around this topic. It seems there is no real consensus about frequency – most I speak with think weekly or fortnightly are acceptable. But is there an optimal time interval? Arguably, one of the most remarkable studies ever undertaken on this subject was conducted by a New Zealand team, published way back in 1999 (Hagemann et al., Molecular Reproduction and Development 53: 451-8). They determined the day of ovulation by ultrasound (Day 1 of the oestrous cycle, which equalled the first day when the largest (dominant) follicle was no longer observed), then slaughtered 50 cows divided over 4 different days – Days 2, 7, 10 and 15 of the cycle. This coincided with 2 follicular waves (Day 2 and 7 were the 1st wave, Day 10 and 15 were the 2nd wave), but also when follicular growth (Day 2 and 10) or when a dominant follicle (Day7 and 15) was present. For each pair of ovaries collected, they hand-dissected every follicle equal to 3mm or more. This enabled accurately sizing follicles, followed by IVP from the released COCs, plus a multitude of follicle stage and outcome matched material was collected. The results were very clear: aspiration of the larger follicles produced more blastocyst stage embryos, but importantly, the presence of the dominant follicle considerably suppressed blastocyst development. This was further supported by investigating the level of apoptosis in small follicles, demonstrating that the presence of a dominant follicle increased cell apoptosis in such (subordinate) follicles. As follicle aspiration triggers a new follicle wave to be initiated, perhaps a 10-day gap should be considered as optimal!! These cows were 2-wave cycling cows, of which the majority of cows are, but there are also 3-wave cycling cows that would push this timing out a little. However, I’d be glad to hear if someone uses such timing and if they believe it has a positive impact!