You can check plugs each morning. A vaginal plug is the congealed seminal fluid from the male left in place to prevent the female from mating with another male. Females typically don't allow mating unless they are in estrus so the presence of a plug is a good indicator of a mating event, which often leads to pregnancy (though not 100%). Females cycle every 4-5 days so I usually pair the male and female on Monday and check every morning until Friday. If she hasn't plugged by Friday I separate the pair and start again on Monday. This way I don't have to come in early on weekends and I am more likely to get a litter during the week, but you can time it to what fits your experiments best. It's best to check plugs first thing in the morning because the plug eventually falls out. Sometimes the plug is obvious as a white blob blocking the vaginal opening, and sometime it is deeper, but using a pipette tip you can gently probe the area and if it feels crusty and stiff, then there is probably a plug.

Of course, what Liz said is perfectly fine, nicely explained for a new person. Additionally I would like you to know that morning checking is always better but to check once in the evening too is not a bad idea as vaginal plugs generally last for 16-18 hours and  if by chance mating happens during afternoon (early evening) I may miss to see it in the next morning. But in my experience, in some of the rat strains vaginal plugs are difficult to notice due their short duration or something else then double checking one can do vaginal smear check if it has sperms or not. Otherwise in general vaginal plugs are very prominent that you can not miss it. I hope it helps.

All the best to your experiment

The answers given by Liz and Sumana are appropriate.  I want add something.  You can make a vaginal swab with the help of very small piece of soft sterile cotton and a small plastic sterile stick,  dip it in sterile normal saline.  touch it at the surface of vagina at place obtained matter on glass slide and observe it under the microscope for presence of sperms every morning.  I have done it and found useful. 

Thank you all 

To the answerers: Do you perform a staining of the vaginal smears or do you simply check them under light microscopy? Do you have images you could provide? Thank you! Laura

To Laura: you can use crystal violet stain to perform that. I have done this experiment and it was very beneficial for the determination of the phase of estrus cycle in rat females, depending on a paper on JOVE (attached link)

http://www.jove.com/video/4389/performing-vaginal-lavage-crystal-violet-staining-vaginal-cytological

Thank you, Abdullah!

learnt alot by reading the comments '

learnt alot by reading the comments '

Please any method to identify weeks of pregnancy in a rat

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