It usually depends for me. This last cycle I did them a little early and it was a good thing because I O'd early. It really depends on how I'm feeling. I would say start anywhere after CD 7 or so. Have you been charting? If you have then I would base it on when you usually O.
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CD 8/9 I'm thinking I O early....still trying to figure it out tho. I had a very faint OPK on CD 12 (which means it's neg.) but the next day nothing on the OPK and nothing the rest of the week.
Re: OPK's
BFP #1 March 24, 2010; missed m/c May 26, 2010 @ 12w 4d; D&E May 28, 2010
BFP #2 Oct 20, 2010; My little boy was born on July 5, 2011
BFP #3 April 30, 2013; Chemical Pg May 5, 2013
BFP #4 Aug 22, 2013; It's a boy. Loss discovered at 24 weeks on Jan 15, 2014 (cause CMV virus)
BFP #5 April 6, 2014; missed m/c May 15, 2014 @ 9 weeks; Misoprostol May 15, 2014; D&C May June 3, 2014
I usually start around CD 15 and use them until I have a clear temp. shift.
Depends how long your cycle is though. My cycle is 32-35 days and I've O'd anywhere from CD 17-CD 22
IVF#1 Jan/Feb 2011 Lupron Follistim 1/29
ER 2/10 28 Retrieved 21 Fertilized w/ICSI
ET 2/15 Transferred 2 beautiful embryos, 10 frosties
Beta #1 2/24-141~Beta #2 2/28-1007
1st u/s HB 116 2nd u/s HB 133
CD 14 or 15, but only because I have long asss cycles.
Either CD 8/9 or first sign of EWCM.