How accurate is ~28 week ultrasound?

[dovey]

I wasn't going to get an ultrasound because I felt it was unnecessary. But now I realize that I possibly should have had one earlier since my due date is becoming less and less clear....

I don't know when the date of the first day of my last menstrual period was. I know that my partner and I had sex on the 3rd, 4th and 5th of January. I was still bleeding on January 4th. My periods are usually about 5 days long. Then I got sick with what I thought was a bladder infection and we didn't have sex for a few weeks. I had a positive pregnancy test around January 20th.

We figured that I must have had a strange early ovulation in January, and gotten pregnant from that. With that in mind, my midwife calculated a due date of September 28th. But now, I'm measuring slightly small for dates. My fundus is still rising the expected amount between appointments, but it's a few centimeters lower than my midwife would expect. I'm worried that either the baby isn't growing right, or that somehow, the due date we calculated was completely off.

So, we scheduled an ultrasound to help confirm the date. I would rather not have some sort of postdates freak out at the end because we thought I was due earlier than I actually am. Can an ultrasound at 28-30 weeks help to determine the baby's due date?

[dakotablue]

At 28/29 weeks an ultrasound has a range of +/- 2-3 weeks (meaning healthy babies can measure anywhere between 3 behind to 3 ahead with out due date change) so I don't know how much it will help you.

I know my DS measured 2 weeks ahead always and was born via c-sec at 41 weeks. I wouldn't have changed my due date because I knew exact conception (meds to ovulate HCG trigger)

That being said, would it matter? I mean if your dates are off by two weeks couldn't you just wait and see when the baby comes and if towards the end just get an ultrasound to confirm placenta health? I'm not really certain about this but that was my first thought if your midwife would just let you be or just let you change your dates a bit.

Also, No harm in just checking to make sure baby is fine, do one ultrasound then another two weeks later to check for proper growth if that's your concern.

[RedOakMomma]

I would not trust a due date calculation from an ultrasound at that stage.

If they give you the wrong due date (an early one), you could face a lot of induction pressure. It's not worth it.

Go ahead and have the ultrasound, but be prepared to have a discussion with your doctor about the due date issue. Since you only had sex three times that month, I would think that would be a more accurate due date predictor.

[rparker]

Originally Posted by dovey

I don't know when the date of the first day of my last menstrual period was. I know that my partner and I had sex on the 3rd, 4th and 5th of January. I was still bleeding on January 4th. My periods are usually about 5 days long. Then I got sick with what I thought was a bladder infection and we didn't have sex for a few weeks. I had a positive pregnancy test around January 20th.

Regardless of when your LMP was, if you had a positive test on January 20th then you'll be at 40 weeks sometime in late September/early October if not sooner... a positive pregnancy test is a positive pregnancy test; if anything your EDD might be earlier than the one your midwife came up with.

I would only get an ultrasound at this point to check for abnormalities with the fetus. A previous poster already mentioned the degree of inaccuracy for size/dates by the 3rd trimester and you already know that you were definitely pregnant on January 20th. It may be that your baby is just small and/or that s/he is planning on going past 40 weeks or there could be something more serious going on like IUGR... but it's pretty improbable that you're less pregnant than your midwife calculated.

Umm, hope that helps. Fundal height measurements aren't always super accurate either. With my last pregnancy I measured 4 weeks ahead of my EDD (based on both LMP and first trimester ultrasound) and ended up with an average sized baby at either 40 weeks +2 days (LMP) or 40 weeks exactly (ultrasound). So in that case the measurements were just wrong.

[dovey]

Originally Posted by rparker

Regardless of when your LMP was, if you had a positive test on January 20th then you'll be at 40 weeks sometime in late September/early October if not sooner... a positive pregnancy test is a positive pregnancy test; if anything your EDD might be earlier than the one your midwife came up with.

Arrgh. I'm so confused! Wouldn't September 28th be the earliest possible due date if the baby was actually concieved on the 5th? Because of this I thought that the ultrasound could only push the due date forward. Does "date of conception" refer to the date of the sperm and the egg meeting or the date of having sex? As far as I knew, sperm can only live a few days anyway, so it doesn't matter so much, but I'm wondering if I forgot about some day that we had sex.

Although....I'm considering about the pregnancy test. They starting showing positive 10? days (I think) after the baby is concieved. If that is the case then I guess I could narrow the conception date to somewhere between the 3rd and the 10th....Which would place the due date between September 26th and October 2nd.

Anyway, thanks for helping with my obsessing. I'm glad to hear that people have had inaccurate fundal measurements and everything turned out okay. I don't think I'll be pressured for an induction by my midwife based on third trimester ultrasound dates. I've decided I will have the ultrasound today, just to put my mind at ease.

[trimestersdoula]

I wouldn't go by fundal height OR ultrasound at this point.

My first ultrasound was at 11w exactly. Ultrasound said 11w2d so we went with that.
Second ultrasound was at 22w and it said baby measured 21w.
Third ultrasound was at 33w4d and the ultrasound said baby was measuring 31w1d.

I have also been measure WAY ahead since I found out I was pregnant. The reason for the first u/s was because we couldn't find a HB with the doppler at 10w4d after extensive searching. Turns out we were looking in the wrong location (down low) because baby was already up by my belly button.

Since then I have measured about a month ahead, until recently when my weeks started catching up with my size

[Ruthe]

I don't know if this helps at all, but with my previous pregnancy, I had sex on day 8 of a 42 day cycle (like you, still bleeding) then a day or two later, came down with raging mastitis and immediately got started on antibiotics (it was BAD mastitis.) Betweeen being sick as a dog and then having my husband gone on an extended trip, I know for a fact that was the only time I had sex that month and it was WAY too early for me to be ovulating even 5 days out (the longest, I am told, sperm can be viable?) so I theorize either the strong abx, or the illness, somehow triggered ovulation? I predicted Oct. 20 as the latest possible "due date") (based on conception 5 days after sex) and had him Oct 30 (10 pound baby)
SO that isn't necessarily helpful except that I had a similar experience of early-cycle conception with raging illness/abx going on at the time, so there could be a connection there.
Based on a 28 day cycle wheel and my lmp date, my predicted due date would have been Oct 23, just 3 days later than the one we used, but, I had consistantly had 42 day cycles for several months in a row, and pretty much never in my adult life have natural cycles less than 32-42 days so definately something unpredictable and unexpected was going on.

[jdejong]

My two cents - I'm tall and with my first baby, I always measured a bit smaller than the week would indicate (and I was positive about ovulation dates), and if I recall, I was only about 34/35 cm the day before my son was born (just two days before his due date). He wasn't a huge baby either, 6 1/2 pounds, but my midwives never worried - being tall (and quite thin) I just had extra space in my body to 'hide' an average-sized baby. Good luck!!

EDA - And last week, at my 27-wk appt I was 26 cm... maybe the same pattern will emerge in this second pregnancy.

[MaerynPearl]

Sperm can live 5-7 days in ideal surroundings (aka inside of you).

Assuming you got your BFP at 10dpo, you could have easily gotten pregnant Jan. 10th from Jan. 5th sex...

So, in my mind, the LATEST your due date could possibly be is October 3rd.

it is likely before then.

So there should be no post-date freakout since thats less than a week later, especially if your midwife is willing to accept that as your very ultimate latest-you-could-be date.

FWIW my ultrasounds with this little one have been a few days later and later each time... first time being 4 days less pregnant than I am (and I charted, know EXACTLY when I got pregnant) second being 6 days less pregnant than I am and third being about 8 days less pregnant than I am...

however, I tend to have babies who are slightly on the smaller side for their gestational age (but only slightly... by mere ounces... in the low 7 lbs range for being 40+ weeks with DS and 39+ with DD)

I have a friend who almost always measures behind and she generally has tiny babies (low 6 lbs range even at full term) and my mother measured (fundal height) ahead and I was a 9lb baby, on the dot!

[zmom2010]

My personal experience

My FIRST (approx 10 weeks) ultrasound calculated my due date to be 5 days before my calculated due date. Yet, my son was born 11 days later than my calculated due date. After my 22 week US, they made me come back in for 2 more US and diagnosed me with IUGR (intrauterine growth retardation/restriction) and told me there was a possibilty he was going to be an achondroplastic dwarf b/c of his measurment ratios. Then when my son was finally born, he was 21 1/4" and 8lb 1oz.

Point being: I don't trust ultrasounds for anything other than evaulation of pathology (sometimes)!

I hope the remainder of your pregnancy goes smoothly!

[dovey]

Thanks for all the info and experiences. I did end up going in, and luckily the technician said everything looked normal. She also gave me the same due date as my original one based on the size that she measured - which at this point I attribute to chance. I asked about whether the dates were accurate at this time in the pregnancy, and the technician told me that she could accurately determine the date - hmmm.....She was very kind and polite though.