LMP – dating pregnancy

posted 2 years ago in Pregnancy
Post # 2
Member
2696 posts
Sugar bee
  • Wedding: June 2012

I have lots of experience on the small baby front.

First off, to answer your original question, if you don’t ovulate on CD 14 or have a cycle length that is different than the typical 28 days, then yes, that can cause your due dates/baby’s growth to be off. I’m surprised your doctor wouldn’t ask about that. How do you know that you ovulated on day 20? did you chart? If you did, you could provide them with the chart. You could also plug in your own cycle information into a due date calendar to see when you are actually due based off of ovulation/conception.

I’m in my second pregnancy, and both times my babies have measured small. I chart, so I know exactly when I concieved and there was never a question of what my dates were. This has been very important.

This pregnancy, baby started measuring small at my 20w anatomy scan (one week behind). Belly measuring for me didn’t start until 25w or so (they don’t start measuring typically until after 20w). My belly is measuring 2w behind and I have another ultrasound on the baby this week to see what is going on.

In my case, my first baby ended up diagnosed IUGR. We aren’t sure of why. I had other complications with my kidneys, so it’s possible that stress  contributed, and it’s also possible there was an issue with the cord not transferring the required nutrients to baby. We’re not totally sure. Another part of the puzzle is that I just don’t make big babies. Our family has smaller kids, and my DH is small. That being said, my DD was born 5lbs 5oz at 38w 5d, and obviously had been missing out on some nutrition in the womb (she ate SO much after birth that her weight never dropped past her birth weight). The last couple of weeks I had many ultrasounds and she had stopped growing.

Small babies are fine, so long as they keep growing in the womb, it’s once the growth starts that there is a real concern.

A small baby doesn’t necessarily mean a sick baby. You may just make smaller babies…it’s up to the doctors to monitor closely to ensure baby keeps growing at an acceptable rate. Keeping up on kick counts is very important.

Good luck, and if you have any questions, let me know! I know how worrisome this can be, as I am living it right now, for the second time!

Post # 4
Member
2696 posts
Sugar bee
  • Wedding: June 2012

Truffle-hunter:  Never had an amnio or any type of genetic testing. Just the typical anatomy scan and blood work to look for indicators, but all of those came back low risk, so I never required anything further.

Genetic abnormalities are only one reason a baby can be small. There are SO many scenarios that can cause a baby to stop growing, and usually it has to do with the placenta/cord. It’s also most common with mothers who smoke/do drugs, so just be prepared to be questioned on all that too (it can be pretty insulting, lol)

Post # 5
Member
3360 posts
Sugar bee
  • Wedding: December 2011

Truffle-hunter:  Later ovulation would definitely contribute.  Dating based on LMP is not the most accurate method, for the exact reason you stated – if you ovulated later, then your due date should also be later (LMP does assume a day 14 ovulation).  So, for example, if you ovulated on CD 21, then tested two weeks later and got a +BFP, you’d be 4 weeks pregnant – but an estimate based on LMP would put you at 5 weeks.

I can’t comment from experience, but wanted to chime in that your doctor could definitely just be off with your due date calculation!  I hope everything is fine and baby keeps on growing strong and healthy 🙂

Post # 6
Member
2311 posts
Buzzing bee
  • Wedding: July 2011

I think your baby is just small because of some miscalculation of your dates by the doctors. Remember, the doctors are humans too and humans are prone to error, or in their case, prone to whatever they study at Med schools. And that’s why their profession is called a ‘practice’ because it’s not perfect.

I know EXACTLY the date the baby was conceived and also when it implanted. (I had implantation pain and a few drips of implantation bleeding.) So that was 12 days after my conception which can also throw things off by a week here or there which the medical staff does NOT take in to account. They have a blanket calculation for everyone. 

If you’ve had no reason to fear any of those diseases then don’t stress and worry yourself in to a complicated situation. Your baby and you will be FINE!

Post # 8
Member
42460 posts
Honey Beekeeper
  • Wedding: November 1999

Truffle-hunter:  Both of my babies were small with no particular explanation. They were however, both healthy and had no problems.

Post # 9
Member
870 posts
Busy bee

I was told with my first baby that he was small and that his head was too large/odd shape. He came out at 7lbs 5ozs with a perfectly normal head! Unless you drastically stop growing or feeling baby move I wouldn’t worry yourself crazy. Particularly if you are a small person. 

Post # 10
Member
870 posts
Busy bee

Btw I’m Australian and had my belly measured every visit but from memory that was later on. 

Post # 11
Member
1613 posts
Bumble bee
  • Wedding: September 2012

1. dating by your LMP can take into account cycles of various length and different days of ovulation, *IF* your doctor/midwife takes that into the equation (which they should!).  Did they ask how long your cycles are? Are they regular? What day you ovulated? If not, they may have used the typical 28 day calcuation, which in your case is wrong.  If you ovulated day 20, then you would be estimated to be 6 days less pregnant than your due day by a 28 day calcuation (and this would also agree on the date given by your nuchal scan).  Hence, at your “19 week” scan you only would have been 18 weeks.  At that time in pregnancy, the range of error for dating is 7 days, so if you were 18 weeks and the ultrasound said 17 weeks, that “agrees”.  Does that make sense?

2.  a first trimester ultrasound is generally taken to be accurate to about 5 days range of error, in other words if your date by LMP said due June 20 but the ultrasound said June 24, we would say they agree because it’s within the range of error.

3.  with such a low risk of trisomy based on the nuchal scan and quad screen, there would be NO reason to recommend an amnio.  I don’t want to over-step, but that would make me seriously re-consider this doctor.  The risk of miscarriage due to the amnio was far, far greater than the risk of having a baby affecting by one of the trisomies.  Not an appropriate risk-benefit ratio to suggest an amnio.

4. symmetrically small is most commonly due to a normally small baby, aka genetically small due to heritage/ethnicity.  It could be also due to an early infection, but this is much less likely (eg. CMV, herpes, toxoplasmosis, etc).

5. placental issues cause asymmetrically small, ie. the head is normally sized and the body is small.  This occurs later in pregnancy, not typically in the second trimester although I suppose it’s not impossible.

6. an otherwise normal morphology scan, with all the parts present and growing normall (though small) and no other “soft markers” of trisomies is reassuring and again, I really can’t see a clinical reason why they’d offer you an amnio? 

7. we don’t measure the belly (symphysis fundal height) until after 20 weeks.

From what you describe, this sounds like a mix-up of the numbers and a lot of worry over a situation that is most statistically likely to be normal.  Hope things turn out well for you and baby!

Post # 13
Member
11668 posts
Sugar Beekeeper
  • Wedding: November 1999

Truffle-hunter:  

I wouldn’t worry about this at all, right now. Due date based on LMP assumes a 28 day cycle, which it sounds like you don’t have. My due date was Feb 22<sup>nd</sup> based on LMP,  March 5<sup>th</sup> based on my first dating u/s at 8 weeks, and March 1 based on when we DTD/when I think I ovulated. I have 36 day cycles. My baby was born on Feb 20 at 38+1 perfectly healthy and happy. My belly measured “behind” throughout the last trimester, and my doc said that a “normal” window is +/- 2 weeks and is nothing to be concerned about. At the end of the date, the due date is just an estimate. Every baby develops at different rates. Congrats on your baby girl! Enjoy the rest of your pregnancy!

Post # 14
Member
1613 posts
Bumble bee
  • Wedding: September 2012

MrsWBS:  Based on your cycle length, your due date would have likely been March 2 by your LMP but without knowing your LMP that’s only a good guess.  And that date would have agreed with both your ovulation and the early u/s! 🙂

Due date based on LMP doesn’t (or more accurately, shouldn’t) assume a 28 day cycle, the specific calculation is supposed to be adjusted for anyting other than a 28 day cycle.

Post # 15
Member
11668 posts
Sugar Beekeeper
  • Wedding: November 1999

cdncinnamongirl:  SOrry, what I meant is when they first gave my due date (not taking into account cycle length), it was feb 22nd. Regardless, the moral of the story is, you could have mutlipe estimated due dates, they’re all just estimates! Baby comes when he/she is ready!

  • This reply was modified 2 years, 4 months ago by  .
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