Hi,
This can happen sometimes. Presence of sperms is usually a good indicator of pregnancy but sometimes pregnancy might not happen.
This being a rare phenomena there could be some health issues/hormonal imbalance with the rat. This is why we need to do a follow up of the female rat in the period we suspect theres a pregnancy and also check for presence of fetus before sacrificing them.
Hi ashutosh,
Did you check, besides sperm presence, whether the females were in the right period of the estrus cycle? Perhaps you used females finishing the estrus. Males could have successfuly complete the copulation but female's eggs are too old for efficient fertilization. It can also be due to females still too young.
There's many guides available online with good images to visualy identify the cycle from smears of veginal epithelia.
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0035538
Hope it helps
Cheers
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