It depends on how you measure it it's a matter of opinion. 13w3d if you divide 40w by 3, or after 12 weeks if you go by the baby's developmental stages.
I like to go by 13w3d which is today for me! It just makes more sense to me BUT you'll find everyone has a different opinion, 12w, 13w, 13w3d, or 14w are what I typically hear. Pick whichever you like best and go with it!
I always go by 14 wks = second tri and 28 wks = 3 tri. I find it is so easy to get ahead of yourself and then third tri seems to take forever. Someone on my fb thought she entered third tri at 24 weeks, all I could think was that was going to be a long last trimester!
Married - July 2010, DD - April 2011, #2 EDD August 10, 2013
Re: Trimester question
Totally depends on how your Dr. views it. My midwife considers 12 weeks the second tri.
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BFP #1: 12/2009 m/c 1/2010 BFP #2: 6/2010 m/c 8/2010
BFP #3: 10/2011 ectopic 11/2011 (right tube removed, learned left tube was probably nonfunctional due to scar tissue from infection after m/c)
3 failed IUIs, IVF #1: 18R, 12M, 10F, 3 poor quality 5d embryos transferred= BFP #4!!!!!
Betas: 9dp5dt: 64 ~14dp5dt: 91 (expecting miscarriage, doubling time of 236 hours) ~16dp5dt: 200~18dp5dt: 500
First Ultrasound at 6w2d revealed two sacs, only one with a heartbeat
LK arrived after 42 weeks on August 14, 2013! Beautiful, healthy, and happy!
TTC#2: IVF booked for April 2015
Surprise BFP#5 February 19, 2015 EDD: November 2, 2015
Betas: 10dpo: 10, 14dpo: 77, 17dpo: 270
First Ultrasound at 5w1d showed a miracle UTE baby! And right ovary ovulation to left fallopian tube.
JD arrived at 38 weeks on October 20, 2015.
TTC #3: Since October 2017. BFP #6 July 2, 2018 EDD: March 16, 2019 [/spoiler]