To ask for your help explaining this to my boyfriend?

[ufgh]

So, I'm trying to explain to DP about due dates and the fact they're calculated from your last period.
We know full well I conceived 2.5 weeks after that as we hadn't slept together for ages before that.
So now he's confused that we slept together and conceived say 12 weeks ago but baby is measuring 15 weeks.

How do I explain this?

It's a bit weird but it's hardly that difficult to get your head around!

https://www.babycenter.com/404how-is-a-pregnancy-due-date-calculatedd_10338321.bc?scid=Universal%20link

To be fair the hospital system doesn't really make much sense when you first meet it. I like your chap - at least he is thinking about things and trying to understand rather than just accepting it because some said it was so.

Basically, before scans, the only sure date was the last period. So, for standard purposes and not to introduce two parallel systems, dating based on scans uses the same scale, and, effectively, starts at about 2 weeks.

I can't see how "age of the egg" is a good way to explain it, even if you ignore the fact that the egg's been in the ovary. The date the uterus starts shedding its lining doesn't in itself guarantee the date of ovulation.

It seems to me it's much simpler to explain it in practical terms - the date of LMP is in most cases, the one known date in the process. We don't know the date of ovulation and, for couples having sex fairly regularly, we don't know the date of conception. It makes sense to start with the one certain date in the process and guesstimate that conception was likely to be around two weeks after that, given that that is likely to be the most fertile stage.

I agree with a PP. Tell him that prior to scans etc. the only fixed reference point was your last period because it is the only part of the process that is visible. So all the dating protocols start from there.

@ReanimatedSGB Me too! Had my coil out, told it would take me at least 12 months to fall pregnant, dtd 3 days later, and didn’t have sex again when I realised I was late and therefore pregnant. So I could tell the exact day, and time! I conceived- it was still dated from my last period as they all are.

I would go with the explanation @Lweji gave.

It sounds to me that he understands perfectly. He just doesn't understand the logic behind the system of pretending that a baby is two weeks older than it actually it. Imo he's right. There is no logic to it and it is confusing.

Can you just say that the start of your cycle - cycle day 1 is your body beginning to prepare for the pregnancy. The body gets ready to release the egg and prepares a good environment and if you get pregnant this is the first part of the pregnancy. And then the stuff about fertilisation - sperm can live for 5 days, the egg for about 24 hours and if it does fertilise then the time for it to implant can be variable too, usually 6-14 days. Pregnancy is the whole process.

Is he going to be of these men who can’t do night wakenings because he's tired? And can’t change a nappy because he doesn’t understand how it can hold all that pee and what the chemicals are ?

It gets weirder when you have an FET (frozen egg transfer), they don’t count the days before it’s frozen

It's really as simple as the pregnancy being two weeks longer than the conception.

That's it.

You conceived 2 weeks ago? You're 4 weeks pregnant.

You conceived 8 weeks ago? You're 10 weeks pregnant.

You just add 2 weeks (c. 14 days) to the suspected conception date.

It's based on the average period cycle being 28 days, and conception happening exactly in the middle, i.e 14 days (2 weeks) in.

He can be as confused about it as he likes. Refuse to see the logic in it as much as he likes. But it is what it is. 🤷🏻‍♀️

Surely its because, as others have said, most pregnant women know when the first day of their last period was but most will not know the exact day they ovulated, so it's the best, most certain date on which to base a due date and from which to estimate gestation. Everyone who has read anything about this topic knows that during the first 2 weeks there was no embryo, but it simplifies things rather than having to guesstimate ovulation and fertilization (especially given sperm can live for several days, so fertilization may not have happened for days after having sex!).

Surely they use the last period date purely because it is a known date, whereas if you attempted to determine conception but were at it like rabbits for weeks on end you'd have no idea what date to use? They can't/won't then adjust it at the scan as people have scans at different points, and it would completely throw off all the data they use to assess where the foetus is compared to average/where it should be?

It’s funny I was trying to explain this to my husband last week, he really wasn’t getting it, worse though is the fact we already have 2 young children and he’s got a science and masters degree that has a lot of physiology in!! We are planning to have another baby next year but don’t want to end up with an August baby, so I was telling him when we can go for it from to make sure we are into sept. I ended up having to put dates in a due date calculator for him to believe me!!!

It is a weird system, but that's how it is!
Crazy to think that (in most cases) you can't be 1 week pregnant... It jumps from not pregnant at all to 2 weeks!

Also dating scans aren't 100% accurate. My long legged baby was dated as being concieved 1 week before she actually was (even once adjusted for the strange extra 2 weeks), due to her leg measurement!
But both DH and I have long legs and DH is very tall so not surprising that she measured longer than the average foetus!

it's only an estimate isn't it anyway?

For example, could they tell the difference between a big 12 week baby and a small 14 week baby, for example? OK, certain parts of the baby will develop at different rates/rates, so there will be some visual clues, but it won't all happen at exactly the same rate for all babies, so they're just giving you an approximation.

And the length of pregnancy varies quite a bit too. By over a month, according to this.

'only 4% of women deliver when predicted and only 70% within 10 days of their estimated due date'.

Dating it back to the date of the last period is just using one factor that's the most certain. Most women will know that date, especially if TTC, but there is variation in the date of conception and date of egg release.

In the nicest possible way OP, your DP sounds a bit thick.

You need to explain that there are 2 separate things.

One is the pregnancy which is a medical term referencing a biological event. The starting point of which is the start of your menstrual cycle not conception.

The other is the age of the foetus.

I see it as refering to the cycle of pregnancy not the age of the baby. The baby isn't 39 weeks old (or whatever) when it's born, it was born at 39 weeks. The pregnancy cycle begins at the start of the LMP when the uterus clears itself out ready for the new lining. So don't think of it as the age of the little pip inside, it's the length of the biological process that enabled it to be made.

In general, the last period date is used simply because before scans you couldn't verify the other factors eg, ovulation date, how long sperm survived, etc.

This method is still used because its then consistent, as some places do scans differently, some women don't have scans at the right time for whatever reason, etc.

Does anyone know whether all countries date pregnancies in the same way?

Just to really blow your mind - it can move again at the 12 week scan - as the date of last period only works if you have 28 day cycles and so release the egg on day 14.

If (like me) you have 20 day cycles then you release the egg on day 6. So I wandered into my 12 week scan at 12+3 and walked out at 13+4! Meaning that the “first day of my pregnancy” was over a week before my last period! (And it seems right as well - dd was born 4 days early on new dates so 12 days early on old dates and was well over 8 pounds.)

Just stop bringing it up. Honestly, unless you’re constantly looking at ways to show him up for being a bit dim.

He believes you, he doesn’t need to know the ins and outs.

The convention of dating a pregnancy 40 weeks LMP arises from the days before scanning etc when the only thing doctors had to go on was the woman's last period

If he won't hear it from you then a very brief word from the midwife that although technically a baby would have been conceived 13 weeks ago, the scans and due dates from the hospital automatically add 2 weeks on as they date it from the first day of your last period. Whereas ovulation occurs halfway through the cycle (approx 2 weeks after 1st day of last period).

That is why people can be 4 weeks pregnant when they had sex and conceived only 2 weeks prior (or even 11 or 12 days prior if they found out with an early test).

It's the hospital/prenatal system that's confusing him. Your explanation should be enough but the MW will say it officially if he really needs that.