Do I need to go through a mock cycle?

Dear Professor Winston,

My partner and I (same sex couple) have had Intra partner egg sharing and were successful in having beautiful little boy (now 2).

We had 1 embryo frozen and again that was successful, but unfortunately I had a late miscarriage at 20 weeks (post-mortem was inconclusive). Needless to say we were devastated and delivering my baby girl was one of the hardest thigs I have done. My partner (age 35 with very good fertility) underwent further IVF and got 17 eggs, 15 fertilised and 4 made it to day 5 and day 6 embryos.

We used the 4AA embryo as a fresh cycle and unfortunately it was a negative cycle, we then tried a BC day 6 embryo and again was a negative cycle. We have 2 day 6 BC embryos left and are so scared they won’t work. My lining has always been good and measuring around 9.5 for each cycle. My clinic has suggested going through their implantation clinic, this basically involves a mock cycle and taking a biopsy of the endometrium to check it for any abnormalities.

Initially we were very excited for this but have learnt it costs £1550, after spending over 60K at the clinic we are very reluctant to do this if there isn’t much evidence that it is worth it. My question is, have you heard of this treatment? Do you think it would be worth it for our circumstances?

We are just terrified that we won’t get pregnant and won’t be able to give our little boy a sibling.

Thank you so much in advance,

L