Cycle length and length of pregnancy

[yellowflowers]

Hi, I am a bit confused because I've been told that if you have a longer menstrual cycle your baby is likely to be longer than 40 weeks. I ovulated on day 18 of my cycle - I def know this from temping etc. Does that mean my baby will be late? Surely not necessarily as if you have 50 day cycles your baby won't be inside you for over a year. Any light you can shed on this please...

I have a short cycle and dd1 was nearly 2 weeks late. old wife tale?

I am interested in this one too! The due date hospital gave me is according to my LMP, but often my cycles are up to 35 days long (and according to 12 weeks scan, which we had to pay privately for at a different hospital) I am due about 5/6 days later than.

Just worried for induction now!

I think it's to do with when you would have conceived compared to your LMP. They calcuate pregnancy as being 40 weeks after the first day off your last period, so..

For example, I have 28 day cycles, ovulate around day 15 and conceived around day 18. That makes my due date at 40 weeks after my LMP(38 weeks-ish after conception) pretty accurate.

Dsis on the other hand has really long cycles and might ovulate around day 25, and conceive just after. Add 38 weeks to that takes you to 42 weeks-plus since her last period. So a seemingly longer pregnancy if you go off your last period but going from the likely date of conception, not actually any longer.

I think this is why dates are often changed at the 12 week scan depending on the stage of deveopment of the baby- going off LMP dates only has obvious disadvantages.

Hope that makes sense.

Planner, don't worry, induction is always negotiable, especially if you dispute the dates. You can always opt to be monitored regularly rather than be induced...although if you go that far over you might be tempted by induction!

It's probably best not to think of a 'due date' and acknowledge that a normal, healthy pregnancy is between 37 and 42 weeks long. And nobody can force induction on you anyway.

If you ovulate later, then the likely conception date is later, and thus your due date with it. So your due date would be 4 days later than expected.

I wonder if it is also the case that women with long menstrual cycles also tend to have a longer gestational period for their babies, and so typically give birth at say 42 weeks, rather than 40. It is certainly the case for one good friend of mine, but I'm not sure we can extrapolate from one case to the rest of the population!

And my friend who ovulates on day 9 has gone to 41+ weeks with all of her babies, so that cancels out peasandbeans' friend.

The "40 weeks from LMP" dating works by saying "38 weeks from ovulation" to which they then add 2 weeks because the average woman will have an average cycle of 28 days and will ovulate on day 14. Given most women don't know when they ovulate but do know their LMP they use the averages to get a starting point.

Combine that with an average gestation being between 37 and 42 weeks, and you start to see why the most important bit of an Estimated Due Date is the word Estimated...

If you have a 50 day cycle then its reasonable to assume you ovulate around about day 36 (the luteal phase, which is the bit between ovulation and menstruation, is much more predictable) - 36 days is about 5 weeks, so you're EDD should be recalculated to 43 weeks from LMP (38 weeks from ovulation + 5). However, this adjustment relies on whoever's doing the calculation both understanding what they're trying to do and being prepared to believe that a woman might understand her own menstrual cycle. Some MW's are better at this than others...

Its worth bearing in mind that you can write what you like on your hospital notes. If you're confident that the EDD based on LMP+40 weeks is incorrect then you are at perfect liberty to adjust it. You are also free to decline an induction at any point - that often goes more easily if you can give a point at which you will accept induction (so if you're happy to be induced at EDD + 10 days and can show why you're EDD is later, you're not actually disagreeing with them...).

Hope that makes sense (sorry it got so long) and fingers crossed you all have straightforward pregnancies :)

40 weeks is calculated from the date of your last period. But this assumes that your cycle is 28 days long. Most are not.

Your Luteal Phase (LP) is the length of time from ovulation to your period. This rarely varies significantly and is usually 14 days, although can vary from 10 - 16.

The bit that tends to vary cycle length siginificantly is the phase prior to ovulation.

So if you know when you ovulated, count forward 38 weeks. If you know your cycle length count 36 weeks forward from the date your period was due.

And remember anything between 37 - 42 weeks is normal so not worth stressing over a day or two

Isn't it slightly irrelevant how they calculate? Yes, of course the lmp provides only an estimate (hence the scan providing a revised date in many cases).

I thought the op was specifically asking if cycle length affected expected gestation. IME, no, in most of my friend's experiences, no but I don't know if there's been any research into it.

thank you. My concern is that hospital think baby is due earlier than I think it is (just by three days) and I don't want an induction earlier than necessary. But because they date they gave me for induction if I am late is ten days after their due date that would be just before xmas and I wonder if I can say no which might mean instead that it would be born or induced on xmas day - I don't mind this but is it unfair given there would be fewer nurses and doctors on duty etc to refuse an induction because I think they are too early and give them more work to do on xmas day or boxing day?

Hopefully it will come on the due date that I think is right anyway.

My cycle is long, around 37 days. I ovulate on day 6/7 though so the date i gave myself for my Ds was far more accurate than the one the hosp gave me! I was induced at 13 days over their date, but i'm rather glad as the placenta was starting to shut down.

As for Xmas induction, They don't! DD1 was born on dec 28th, exactly 2 weeks late because of this rule :o

okay that is good - so if i refuse it on 22nd 910 days late by their notes or 7 by mine) then they wouldn't induce until Dec 27th or so anyway? Which by working out is nearer 2 weeks overdue anyway.

Remember though that the time from fertilisation to implantation can be a bit variable - I was temping and so know I ovulated on day 17 so when I went for a scan at 8 weeks LMP expected to be put back a few days, but actually was put at 8+1 (and I made them check the measurements a few times!) - all I can think of is that it must have implanted quickly. So you won't necessarily be due 4 days later than your LMP-based due date.

what date did your can give you sorry to say but that tends to be more accurate than LMP

I have a 30 day cycle... I did a calculation on line, however, after my early pregnancy scan, my obstetritian changed my due date by a week longer than the original calculation had suggested.