Idally speaking, no form of medication should be used at least in the first trimester of pergnancy to avoid any chances of any congenital anomalies, except the Folic acid tab daily. But as you have already been exposed to nitrous oxide which though dissolves very fast in the blood and cause short anaesthesia, but also is eliminated sooner from the system, so it is unlikely to cause an effect.
However it is important to establish a fetal cardiac activity by 6-7 week to be sure if it hasn't caused any possible defects which may sometimes land up in missed abortions. Also antibiotics are to be avoided in the first trimester which you may have been given with your dental treatment.
In studies long exposures on nitrous oxide have been seen to cause spontaneous abortions in many but with short exposures likelihood is less. So, it is advised to stay informed and follow antenatal protocol which can be explained by your physician in detail, and hope for a safe pregnancy.
I hope i have answered your query in detail,
wishing you safe pregnancy , regards
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