It's your pregnancy, it's up to you. I found I was pregnant at 5 weeks and told my husband and my mom the day I found out but we waited until about 8 weeks to tell everyone else. Tell people whenever you feel like it. As ProudMumToBe said a lot of people do wait until they're about 12 weeks but I think it's just a superstitious thing.
I told my family at 12 weeks. There's a history of miscarriage in my family (I don't know if it's genetic or not but it's a lot. My mom had 2 miscarriages, my grams had 6, 2 of my aunts each had 4 and 1 aunt had 3.) and I didn't want to tell them and have something happen. Luckily nothing did and everyone was super excited.
It's up to you. At my 1st visit that we saw the h/b (6w) my RE said that although the h/b was strong, based on my age (I am 38), there was a 40+% chance of m/c at least one twin. We saw the h/b a couple more times and me and my DH felt more secure so we told the families at X-mas (I was 9.5 weeks). I was already beginning to show and everyone figured out I was pg anyway - the twins were a huge surprise, though. If you feel confident and are comfortable sharing the news, then go for it.
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