Chet - posted on 09/11/2014
Even if this mom is trying to figure out the father, why does it matter? The information is the same no matter why you want to know. The only people who have exact information regarding conception are people who've been monitored very closely with infertility treatments like IVF.
There are lots of people who would like know their exact conception date though, not just moms trying to figure out the father....
A mom may want to know the exact date because the baby isn't showing the growth they expect. For example, if you go for an early ultrasound and they can't find a heartbeat. It's possible that the pregnancy isn't progressing and you are going to miscarry, or you might have conceived later than you thought and the baby is fine. Usually they'll do blood tests and a second scan to see which it is, but that takes time. Even with tests or scans that happen later in pregnancy a conception date that's off can mean that bad news isn't necessarily bad news.
An exact conception date can also help to determine the exact date of viability - when the maternity ward will take you and try to save your baby in the event of complications versus when you have to go to the regular hospital and be treated for miscarriage. Since more and more pre term babies are surviving before the official date of viability exact dates can matter.
The other common reason moms worry about their exact dates is because of interventions during labour and delivery. Many providers won't allow home births too far beyond 40 weeks, or will insist on induction at a certain point. A generation or two ago it was pretty common for moms to go to 42 or even 43 weeks. Now a lot of mothers are hard pressed to be left alone past 41 weeks, and will scramble for any evidence that they should be given another day or two.
Of course, trying to figure out who the father is when there are multiple candidates is another reason. But why bring that up if the original poster didn't?
Chet - posted on 09/10/2014
Even if you know when you ovulated, and when you had sex, you still don't know the exact time that the sperm and egg came together. You could still be off by a day if you only had sex once. If you had sex multiple times, you wouldn't know which time the sperm was from.
Being off by a day doesn't matter for due dates though. Only for things like early ultrasounds if you're looking for a heartbeat.
Chet - posted on 09/10/2014
The first day of your pregnancy is counted from the first day of your last period. In a 40 week pregnancy you aren't pregnant for the first two weeks. Those two weeks are when the egg is being prepared and released.
It's almost impossible to figure out exactly when you conceived. Sperm can live for up to a week (though 3 to 5 days is typical). And the egg is available to be fertilized for about 12 to 24 hours. So you might have sex on Monday, release an egg on Thursday and conceive on Friday.
A due date is really just a best guess. If your cycle is 28 days long the counting for weeks from the start of your last period is pretty reliable. A lot of the online pregnancy calculators allow you to enter different cycle lengths.
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