Only about 5% of babies will arrive on their due date and focusing on this single date can make the end of pregnancy quite stressful. It may be more helpful to be prepared for you baby arrive some time after 37 weeks, and to focus on 42 weeks as the time by which you have a good chance of having given birth.
More than 90% are born two weeks either side of the predicted date. But, as noted above, only 4% (or 4.4%, ignoring pregnancies with complications etc) are born on the predicted date itself - in other words, the chance of this happening is less than one in 20.
Between 22 0/7 weeks and 27 6/7 weeks of gestation, ultrasonography dating has an accuracy of ± 10–14 days 19.
First babies are less likely to be “on time” at 39 weeks, and more likely to be a little late, between 41 and 43 weeks. Among full-term pregnancies, first babies are born about 1.3 days later on average.
It is very common when scanning an early pregnancy to find that the due date does not match the menstrual history. Sometimes the dates can be more than a week off and sometimes even as much as 4 weeks.
The most accurate way to date a pregnancy is with an ultrasound done between 6 weeks and 8 weeks, 6 days, according to Dr. Michael Cackovic, MD, an OB-GYN with The Ohio State University Wexner Medical Center.
A normal cycle can be between 21 to 35 days, and some people can even have cycles that range outside of that. Regularity isn't that common, so there's a fair chance that the calculation will be off, but the last menstrual period measurement is still used because it's considered fairly reliable most of the time.
An ultrasound is actually the most accurate way to date a pregnancy because all fetuses grow at a consistent rate during the first trimester and early second. In other words, if your baby measures 9 weeks 2 days when you have your ultrasound, that's how far along you are, no matter when your last period was.
As pregnancy progresses, the accuracy of an ultrasound for predicting due dates decreases. Between 18 and 28 weeks of gestation, the margin of error increases to plus or minus two weeks. After 28 weeks, the ultrasound may be off by three weeks or more in predicting a due date.
The third trimester is the least accurate time to date a pregnancy. Estimates based on an ultrasound can be off by as much as three weeks, so doctors rarely adjust dates during the third trimester.
Your weeks of pregnancy are dated from the first day of your last period. This means that in the first 2 weeks or so, you are not actually pregnant – your body is preparing for ovulation (releasing an egg from one of your ovaries) as usual.
After the infant is born, there are a variety of characteristics that can be used to estimate the gestational age. It is possible for gestational age to be inaccurate by up to 2 weeks, even with an accurate LMP date confirmed by other tests.
“If dating is only based on the last menstrual period and a later ultrasound shows a discrepancy, then the due date may be changed,” Lamppa says. If your due date is confirmed by an ultrasound in the first trimester, it shouldn't change as the pregnancy progresses, even with additional ultrasounds, she says.
The reason why the baby is overdue is usually not known. Sometimes it is because of a genetic predisposition (hereditary). Women who have already had a baby that came much later than their due date are more likely to have an overdue baby in future pregnancies. Being born after the 40th week only rarely harms the child.
Most babies (91%) in Australia are born at term (37–41 weeks).
You at 40 weeks
Pregnancy normally lasts about 40 weeks – that's around 280 days from the first day of your last period. Labour usually starts a week either side of this date, but you might go overdue. Your doctor may suggest inducing labour – it's your choice whether to have this or not.
81% having their first baby went past the due date. 61% of second, third or later pregnancies went past the due date. Fetal surveillance begins to be recommended.
Third-trimester ultrasound (beyond 28 0/7 weeks) is the most inaccurate method for pregnancy dating with an accuracy of +/- 21-30 days[1]. One major concern with third trimester dating ultrasound is underestimating the gestational age of a growth-restricted fetus[1].
Most pregnant people—about 80%—deliver sometime between 37 and 42 weeks, and about 11% deliver prematurely. While there aren't any hard and fast rules as to why some people deliver before their due dates or why others deliver later, there are a few things these groups have in common: Multiples.
Very active baby before labor
Some women experience their baby moving a lot in the run-up to labor. One theory for this is the increase in Braxton Hicks contractions. As your body prepares for labor and birth, you might start to experience a greater frequency of Braxton Hicks contractions.
The baby will keep moving until the labor begins, and this movement will continue during the early labor. However, the movement pattern may change. Instead of kicking the womb, the baby may squirm or shuffle. The feeling of the baby's movement provides assurance that the baby is doing well.
According to the results of the present study, walking during late pregnancy could be an effective, safe, and acceptable method to achieve cervical ripening and spontaneous onset of labor.
Ovulation usually occurs about two weeks after your menstrual period and lasts for two or three days. This means you can sometimes determine when you conceived based on your ovulation cycles — simply determine when your last menstrual period was and add approximately two weeks.
Missed period
The first sign of pregnancy is usually missing a period, about 2 weeks after you've conceived. This isn't always reliable and if your periods aren't regular you might not notice you've missed one. Some women have a bit of bleeding as the egg embeds. Many women also experience tender breasts.