13 weeks - 1st or 2nd trimester?

kgauntt · November 2011

Is 13 weeks considered part of the 1st or 2nd trimester? My OB nurse told me that 13 weeks was 2nd, but I've been reading on here and it sounds like 13 weeks is technically the last week of the 1st and 14 weeks is the start of the 2nd. I know it's just one week difference but I'm pretty anxious to get out of this 1st and move on to the 2nd (as we all are).

CC.to.be · November 2011

Technically, if you were to divide out all the weeks 13 weeks and 3 days would be 2nd tri. To me thats just splitting hairs. I say 13 weeks is 2nd tri.

FyreFlyeRush · November 2011

This has always driven me crazy, and is why I like What To Expect When You're Expecting. It always tells you the week you're actually IN, not the week you just completed. According to WTE, it's broken up like this: 13, 13, and 14. And, keep in mind at 13w pregnant, you're entering your 14th week. So, 12w6d marks the last day of the first trimester, 13w0d marks the start of the 2nd, 25w6d marks the end of the 2nd, 26w0d the start of the 3rd, and 39w6d would be the end of the 3rd. My OB says 13w is the 2nd trimester.

kgauntt · November 2011

Thanks!

valroberts2002 · November 2011

That is helpful! Thanks for breaking it down. I was curious too.

fyziksgirl · November 2011

There are about three ways to count trimesters out there, so it depends who you ask/where you look. One way says the first trimester ends at the end of week 12. One says it ends at the end of week 13. And one says it ends at 13w3d. The first way is more to count by development, the last way is technically 1/3 of a pregnancy. I'm not sure the reasoning behind the middle way. In reality it's just how we like to think of it and the baby has no idea when they've passed a "trimester," so pick what feels right to you and go with it

kredlund@comcast.net · November 2011

I have a book and they say 13 weeks is the end of our first trimester. 14 weeks is our second. :-) I think I am slowly starting to feel better. :-)

Juleslynn79 · November 2011

I would say 13 weeks is close enough to say 2nd tri. For some reason, my Pregnology ticker says I'll be in the second tri this Thursday, when I am 12w6d. I am not sure why.

JNoelleE · November 2011

the what to expect book says you enter your 2nd tri when you are 14 weeks. You cant just divide the weeks by 4 to figure out how many months you are, because only one month (feb) has 28 days..(some people say pregnancy is 10 months not 9, but that is only true if they are going off of lunar months, and counting each month as being 28 days long.) So technically your first trimester is up until your 3rd month is completed. And once you hit the 14th week, you are just entering your fourth month. So I consider 13 weeks to be the end of your 3rd month and the last week of your first trimester.

DCsara · November 2011

I understand the urge to be able to say you're out of first tri, but honestly it's not like the risks change drastically over the course of that week! Just relax and consider you're in second tri if it makes you feel better.

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