ultrasound date versus LMP date

[porcelina]

I'm now 38 weeks pregnant, and now this question has become relevant! My due date according to LMP (Dec 14) is Sept 20. However, they did a few early ultrasounds (to rule out ectopic, around 6w4d and 7w4d), and the estimated dates were Sept 26 and then Sept 27. However, my periods are quite regular, usually 28-29 days, though only occasionally a day or 2 longer. I find it hard to believe I ovulated a full week later than usual. The doc at the time said they wouldn't change the date unless it was 2 weeks off.

I then transferred to birth center. I asked my midwife which date they might go by, when it comes down to determining how long they would let me go (cutoff is 42 weeks). She said since I had regular periods, they would stick with the LMP date. So now of course I'm starting to wonder, what happens if I go pretty late with this guy?

In your experience, has the early ultrasound been more predictive?

My first son was born at 40w1d by LMP. I also had an ultrasound at 6w5d with him, and he was measuring about 3 days ahead (not behind, like this guy). However, I also had IUI and other minor hormonal interventions with him, and so we knew the likely date of conception -- according to that due date, he was 4 days late. The current pregnancy happened completely naturally.

[finnegansmom]

Last menstrual period doesn't tell you when you ovulated. And just because your cycles are totally regular to the 28 day cycle doesn't mean you ovulated on CD14 or 15. Are you charting?
If it were ME I would go with the latter US date.

My second was born at 41 weeks using the LATEST date I could muster for ovulation. Had I used my LMP I would have been put over 42 weeks. And my cycles were totally regular at 29/30 days BUT I was ovulating at CD20/21.

My order is:
Confirmed ovulation trumps all
Good Early ultrasound dating trumps LMP in irregular cycles
For you, regular cycle but not knowing Ov in this case I'd use the US date
And for the record I'm a believer in NSTs after 41 weeks to check on things like fliud level etc.

[MaerynPearl]

With DS, we went with LMP. Ultrasounds tended to show him as a bit further behind and he was born at 40w4d after an induction (for an earlier problem with the pregnancy)

With DD, I was not charting and went with the latest US date I got. She ended up being born a week before then (granted I did not know LMP) but at least I was not being forced into an unnecessary induction!

This time, I was charting. I had to actually adjust my LMP and my ultrasounds have only ever been a couple days off.

I would definitely trust an early (6w/7w) ultrasound over LMP if you do not know for sure when you ovulated.

[kalamos23]

I would definitely trust early u/s as well - at 6-7 weeks there is MASSIVE growth difference (baby grows 1mm a day during those weeks) and so they are usually pretty accurate. For instance, I got pregnant directly after a m/c with this one, and ovulated on CD24. I don't normally ovulate on CD24, but obviously the m/c messed things up a bit. I did take OPKs so I knew generally when I ovulated and the early u/s confirmed the ovulation date. Otherwise, they would have thought I was 10 days behind, kwim?

[kalamos23]

Oh, when did you get your BFP in relation to your cycle? If you got it on the first day of missed period, I'd be more likely to go with the LMP, but if you got it 4-5 days later, than early u/s.

[porcelina]

Originally Posted by kalamos23

Oh, when did you get your BFP in relation to your cycle? If you got it on the first day of missed period, I'd be more likely to go with the LMP, but if you got it 4-5 days later, than early u/s.

Hmm, good point. I got BFP at cd29, I think, but no, I was not charting.

[MegBoz]

Actually early US are known to be pretty accurate for dating (whereas latter term US are NOT.) Since it works in your favor anyway, I'd go with the US date. Hopefully if you argue with the MW at the BC, they'll give you the slack.

Originally Posted by porcelina

I find it hard to believe I ovulated a full week later than usual.

My cycles are also normally 27-28 days. But I conceived this baby on CD 22!! MUCH latter than I normally ovulate. I wasn't charting, but I know because that cycle we only had sex on day CD7 and again CD 22 - so it had to be then!

Did any stressful event take place? Were you traveling? Anything going on with your health? Like an illness or something? Extra exercise, less sleep? All those things can delay ovulation later than normal.

We didn't have sex --and my ovulation was delayed-- because our dog became horribly sick all of a sudden & was hospitalized for 5 days with an auto-immune disease. She's fine now. Funny though how I feel like I'm PG because my 4-legged little girl almost died.

I agree with the 'order' of accuracy posted by finnegansmom. Of course, I didn't have confirmed ovulation, but knowing we didn't have sex between CD 7 and CD 22 is pretty darn good confirmation, IMO.

[CherryBomb]

I have a 6 day discrepancy between my LMP and early u/s dating. I was charting (Creighton Model), but it was also just 2 cycles after a second trimester miscarriage, so my cycle was very irregular. I still think my edd is more like the 9th or 10th as opposed to the 13th that the u/s gave, based on when I ovulated and likely conceived (there was only 2 chances for conception, and one was like 9 days after I ovulated, so I'm assuming the act of intercourse that resulted in this baby was the one the day after ovulation!)

[pollyanna123]

I would definitely try to go by whichever date is later. IME, my early u/s dates have been extremely accurate, so I'd definitely trust that more in your situation.

[Adaline'sMama]

My LMP date was March 25 2010. My OB swore that my due date (by ultrasound) was April 13. That is almost a 3 week difference. I never complained about it, because I felt like if they kept the later date, there would be a lot less chance of induction

I had only had sex 4 times during my whole menstrual cycle, and it was all in a two day period. (we were on vacation..with family and that was our two days alone). So I pretty much know the exact day it happened.

DD was born on March 20, to a very surprised OB who thought she was a month early.

[MaerynPearl]

Originally Posted by Adaline'sMama

I had only had sex 4 times during my whole menstrual cycle, and it was all in a two day period. (we were on vacation..with family and that was our two days alone). So I pretty much know the exact day it happened.

well sort of... sperm can live in you up to 5 days so it could have happened up to 5 days later, but still...

[kalamos23]

Adaline's mama - was it an early u/s or a later? That is odd!

[pregnant@40]

The population mean time of ovulation is mid-cycle, but obviously there is variation from woman to woman and from cycle to cycle. I've read that delayed implantation can also affect gestational age. Thirdly, there is natural variation in fetal growth.

Regardless, you're only looking at a one week discrepancy. I've recently read that babies who have the full 40 weeks in utero have a decreased chance of cerebral palsy, so what's the harm in waiting, huh? I believe the time to consider inducement is at 42 weeks. Whether you go w/ LMP or the u/s, this child will get 40-41 weeks in utero, and that's great!

[Aliy]

i'd take the later date gives you more time before you are " late"

[SumnerRain]

I was charting, so I know when I ovulated. Normally I ovulate somewhat late, CD 17 with DS3, CD 24 with DD, and CD 18 with this one. I had early u/s with this one and DD and both times by LMP I measured WAY off, but with the early u/s, I measured exactly to the day of when I ovulated. So for ME, my early u/s's were MUCH more accurate than LMP.

Also, some women have short luteal phases (the time between ovulation & menstruation) so even if you have a 28 day cycle, it does not mean you ovulate on day 14, it could be anywhere from day 12 to day 20.

Even though this time it was only a few day difference, I was so upset when the OB I saw at first wanted to keep my due date at December 24th by LMP, when I knew when I ovulated. Even after early ultrasound confirmed I was right and my due date was December 27th, he refused to move it. Knowing that the time after 40 weeks is when OBs get induction happy, I was thrilled when the M/W I transfered into agreed with my Dec 27 date. Go with the u/s date. Buys you more time, and IME is more accurate.

[porcelina]

Thanks everyone for your thoughts! I'm going to talk about it at midwife appt this week. In the meantime, just have to get used to the idea of being a little less far along!!

[myk]

my LMP date was never accurate. with my first, i ovulated on day 7, a full week before the assumed ovulation date. with this one i ovulated on day 12. to save the hassle i just gave the LMP date two days off i knew it because of charting (and OPKs on the second). i had more than one early u/s for both and they always matched my actual ovulation date.. so i always consider that pretty accurate. until about 15 weeks that is - both my babies start measuring larger at that point. apparently i make big kids

[2xy]

Early ultrasound is more accurate than LMP or late ultrasound.

In the early weeks, the embryo is busy with cell division. That's where its growth comes from. All human embryonic cells divide at about the same rate, meaning that your embryo and my embryo and your neighbor's embryo all look pretty much the same at 6 weeks, or ten weeks, or what-have-you.

All women do not experience a perfect 28-day cycle. And later on, babies' sizes begin to differ due to genetics and environment.

[Adamsmama]

In a similar predicament. By LMP my EDD is 11/17, I knew I O'd late, though...so I was pretty sure my due date was more between 11/26 and the latest was 11/30 ... by all the signs of O. I got my first very faint BFP by my latest date at 10 dpo ... and my confirmation BFP at 13dpo ... that seems pretty accurate. When I requested an early u/s it came back with an even later date...which would mean my first BFP happened at 7dpo and my ultra dark BFP at 10 dpo. I refuse to believe that. I thought I read there could be a 7-10 day difference even on early u/s. Also, I had another u/s (to rule out twins because I was measuring very large) at 13 weeks and the baby was measuring exactly a week bigger and I've had to have a few more u/s and each time baby measured 1+ week bigger (arms/legs are always 1 week bigger exactly ... but other parts were a little larger, etc.).