I've always been kinda confused about calculating the number of weeks pregnant. It looks like most people (and doctors) calculate based on date of last period, versus ovulation/conception, right?
So if the first day of my last period was 11/12, but my ovulation date was 12/1 (18 days into my cycle), am I technically 7 weeks PG?!
hostess / wonderful watermelon / 39513 posts
I think most go by LMP but it might change at the first ultrasound based on measurements. My LMP due date is 4/26 but my OB pushed it back to 5/2 based on the measurements at my first ultrasound. The OB who did my NT scan agreed with the adjusted date when I had that scan at 12 weeks.
GOLD / watermelon / 14076 posts
I believe LMP is based on ovulation on CD14. If you O'ed on CD 18, you'd be due a few days later than your EDD based on LMP. Most doctors use LMP because it's easier and most women don't know the exact date they ovulated.
wonderful clementine / 24134 posts
Lmp! Hard to believe you are already 7 out of 40 weeks through!
pomegranate / 3438 posts
When I did my IUI I didn't trigger until CD 22. At my first OB appointment the nurse wanted to use my LMP. We went round and round on that since I knew exactly when I ovulated so she wrote down my LMP and ovulation dates. When I got my dating ultrasound my date was right.
hostess / watermelon / 14932 posts
Exactly what PPs said..with a "normal" 28 day cycle, you are considered 4 weeks pregnant the day of your missed period.
I ovulated 2 days late, so while my practitioner said I was "10 weeks" at my first ultrasound, my baby actually measured (and continued to!) 9w5d. She was born "2 days late," but in actuality, exactly at 40 weeks!
They might adjust it once you have your first appt 
pomegranate / 3398 posts
We did LMP but with the first ultrasound it was moved up 4 days so none of those really lol
GOLD / wonderful coconut / 33402 posts
They went by my last period date. But would have changed it depending on how the baby was measuring but she was right on track.
pomegranate / 3329 posts
This whole thing just confuses me so much!
What do they do for someone who doesn't have a regular period? They make you guess when your last one was. Mine was around the middle of October, so that would make me 11w2d.
Who knows though I could've ovulated in November, so for this instance we have to wait until our u/s on 1-10.
wonderful pomelo / 30692 posts
They usually go by the date of your LMP, but they'll change it if you're measuring a week or more off from that (at least that's what my OB's office does). Ovulation is supposed to be the "2 weeks pregnant" mark, so feel free to count from that since you know when you ovulated, although your doctor may or may not agree!
cantaloupe / 6730 posts
@StbHisMrs: If you don't know, you'll often be sent for a dating ultrasound. I know that I ovulated 2 weeks late, so I made them pretend that my LMP was 2 weeks later than it actually was.
pomegranate / 3414 posts
At my u/s they estimated I conceived on a Friday but my LMP started on a Sunday. The u/s tech said that unless the dates were more than a week off, they typically use LMP.
wonderful cherry / 21504 posts
Mine went based on LMP, but also asked if my cycles were regular. I was charting and knew I ovulated on CD17, but those couple of days only changes my due date by a day or two. They did roughly go on the O date though, I think. I didn't have a dating u/s since they said my dates were pretty clear. If my cycles had been more irregular, they would have had the dating u/s to confirm.
grapefruit / 4669 posts
Mine is based solely on an early ultrasound (long cycles and lazy charting).
wonderful olive / 19353 posts
My OB went by my LMP even though the ultrasound tech dated me slightly differently.
papaya / 10570 posts
I was charting and, based on my ovulation date, I'm 10 weeks today. However, based on my LMP, I'm 10w5d. My midwife is currently using the LMP date but once I've had my 12 week scan they'll adjust the date depending on how big the baby's measuring.
blogger / pomegranate / 3300 posts
They count the weeks from you last period at first by then use the date form your first ultra sound. When I was pregnant with my twins I would only have a cycle every 3-5 months. If they went by my lmp I would have been 18 weeks when the ultra sound measured 6 weeks. The nurse practitioner had sone trouble understanding me at first when I was telling her I was not 4.5 months pregnant
olive / 73 posts
I think if you know when you ovulated, it should be based on that. I know most practitioners use LMP no matter what though, so since I knew I ovulated on CD21, I just told them that my LMP was a week later than it was.
pineapple / 12802 posts
FF told me, based on my ovulation, that I was due August 9. My doc went by my LMP and said I was due August 1.
When I went for my dating u/s baby was measuring at exactly the right size to put me at August 9.
honeydew / 7504 posts
Mine is based on LMP, but even if we went by OD, it's only a day off. By LMP I'm due 1/10; by OD it's 1/11.
ETA: All scans confirmed 1/10.
coconut / 8475 posts
My OB said if my LMP was one week off or more than my measurements then she would ignore my LMP and go by measurements.
My LMP made my DD oct 12
My meas made my DD oct 14
My O day made my DD oct 17
I had him Oct 15th. So it was on point with the measurements basically.
coconut / 8475 posts
@Oceanis723: haha same! I was on the drop shot which only have me periods every 3 months so LMP meant nothing to me! But the cycle I got pregnant was one of the months of the year that I had actually gotten my period.
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