A large body of evidence shows that ultrasounds done in early pregnancy are more accurate than using LMP to date a pregnancy. In a Cochrane review, researchers combined the results from 11 randomized clinical trials that compared routine early ultrasound to a policy of not routinely offering ultrasound Whitworth et al.
The researchers found that people who had an early ultrasound to date the pregnancy were less likely to be induced for a post-term pregnancy. In a large observational study that enrolled more than 17, pregnant people in Finland, researchers found that ultrasound at any time point between 8 and 16 weeks was more accurate than the LMP.
There are several reasons why the LMP is usually less accurate than an ultrasound Savitz et al. LMP is less accurate because it can have these problems:. The authors found that the most accurate time to perform an ultrasound to determine the gestational age was weeks. This was a more accurate result than any of the other ultrasound scans, and more accurate than the LMP. The accuracy of the ultrasound saw a significant decline starting at about 20 weeks.
Using an estimated due date from either the LMP or an ultrasound at weeks led to a higher rate of pre- and post-term births. Ultrasounds in the third trimester are less accurate than earlier ultrasounds or the LMP at predicting gestational age.
All babies are about the same size early in pregnancy. The reverse is also true for babies that will be smaller than average at term—their due date might be moved to a later date. This could be risky if the baby is experiencing growth restriction, as growth-restricted babies have a higher risk of stillbirth towards the end of pregnancy. Because of these problems with third trimester ultrasounds, the American College of Obstetricians and Gynecologists states that due dates should only be changed in the third trimester in very rare circumstances In the U.
In the past, researchers figured out the average length of a normal pregnancy by looking at a large group of pregnant people, and measuring the time from ovulation or the last menstrual period, or an ultrasound until the date the person gave birth—and calculating the average. However, this method is wrong and does not give us accurate results.
This method does not work because many people are induced when they reach 39, 40, 41, or 42 weeks. If you do include these induced people in your average, then you are including people who gave birth earlier than they would have otherwise, because they were not given time to go into labor on their own.
But this puts researchers in a bind, because if you exclude a person who was induced at 42 weeks from your study, then you are ignoring a pregnancy that was induced because it went longer—and by excluding that case, you artificially make the average length of pregnancy too short. There have been two studies that measured the average length of pregnancy using survival analysis:.
In a very important study published in , Smith looked at the length of pregnancy in 1, healthy women whose estimated due dates, as calculated by the first day of the last menstrual period, were perfect matches with estimated due dates from their first trimester ultrasound Smith, a.
In , Jukic et al. This was a smaller study—there were only healthy women, and they all gave birth between the years and However, this was also an important study, because researchers followed the participants even before conception and measured their hormones daily for six months Jukic et al. This means that the researchers knew the exact days that the participants ovulated, conceived, and even when their pregnancies implanted! After excluding women who had preterm births or pregnancy-related medical conditions, the final sample of women had a median time from ovulation to birth of days 38 weeks, 2 days after ovulation.
The median time from the first day of the last menstrual period to birth was days or 40 weeks, 5 days after the last menstrual period. The length of pregnancy ranged from 36 weeks and 6 days to one person who gave birth 45 weeks and 6 days after the last menstrual period.
The 45 weeks and 6 days sounds really long… but this particular person actually gave birth 40 weeks and 4 days after ovulation. Her ovulation did not fit the normal pattern, so we know her LMP due date was not accurate. Women who had embryos that took longer to implant were more likely to have longer pregnancies. Also, women who had a specific sort of hormonal reaction right after getting pregnant a late rise in progesterone had a pregnancy that was 12 days shorter, on average.
Instead, it would be more appropriate to say that there is a normal range of time in which most people give birth. About half of all pregnant people will go into labor on their own by 40 weeks and 5 days for first-time mothers or 40 weeks and 3 days for mothers who have given birth before.
The other half will not. In , Oberg et al. They found that genetics has an incredibly strong influence on your chance of having a birth after 42 weeks:. Overall, researchers found that half of your chance for having a post-term birth comes from genetics.
The risks of some complications go up as you go past your due date, and there are at least three important studies that have shown us what the risks are. In their study, Caughey et al.
However, when the researchers used a statistical method to control for the use of interventions, the risks still increased with gestational age. For more information about meconium, see this article by Midwife Thinking about meconium stained waters.
In this section, we will talk about how the risk of stillbirth increases towards the end of pregnancy. For example, if the absolute risk of having a stillbirth at 41 weeks was 1. But some people may consider the actual or absolute risk to still be low—1. Please see our handout on Talking about Due Dates for Providers for tips on how providers can discuss the risk of stillbirth.
The second important thing that you need to understand is that there are different ways of measuring stillbirth rates. Depending on how the rate is calculated, you can end up with different rates. Up until the s, some researchers thought that the risk of stillbirth past weeks was similar to the risk of stillbirth earlier in pregnancy.
So, they did not think there was any increase in risk with going past your due date. However, in , a researcher named Dr. Yudkin published a paper introducing a new way to measure stillbirth rates.
Yudkin said that earlier researchers used the wrong math when they calculated stillbirth rates—they used the wrong denominator! Yudkin, Wood et al. Instead, we need to know how many stillbirths happen at 41 weeks compared to all pregnancies and births at 41 weeks.
In other words, you have to include the healthy, living babies that have not been born yet in your denominator. When researchers began using this new formula to figure out stillbirth rates, they found something very surprising—the risk of stillbirth decreased throughout pregnancy, until it reached a low point at weeks, after which the risk started to rise again.
This finding—that the risk of stillbirth decreases throughout pregnancy, and then increases sometime after weeks—has been found many times by different researchers in different countries.
In other words, there are higher rates of stillbirth earlier in pregnancy, then they go down until around weeks, after which they rise again. Because the risk of stillbirth starts to go up even more at 40, 41, and 42 weeks, some researchers argue that although 40 weeks and days may be the physiological length of pregnancy, 40 weeks may be the functional length of a pregnancy.
And although the stillbirth rates may seem low overall, if you happen to be a parent who experiences the 1 in event at 42 weeks Muglu et al. Even after researchers began using the new way of calculating stillbirth rates, there was still controversy about the best way to calculate this new formula for measuring stillbirth rates. Other researchers argued that most people and doctors! Hilder et al. They just want to know what the risk would be if they waited one more week until the next appointment, or even a few days.
Boulvain et al. In the end, you will find that stillbirth rates vary from study to study, depending on whether the researchers report the actual stillbirth rate, or the open-ended stillbirth rate. No one can accompany them in the office until further notice. Patient Portal. Pay Bill. That measurement should match your gestational age in centimeters. If you are measuring smaller or larger than what you should be, then the doctor might order an ultrasound.
If the baby is actually smaller or larger than what they should be, underlying issues need to be considered that might be causing the discrepancy in growth. For example, mothers with uncontrolled diabetes Type 1, Type 2, or Gestational cook very large babies. These discrepancies might affect delivery, but they might not. For example, if your baby is consistently measuring small and falling below where they should be, your doctor might decide you need to be delivered because your baby would be better out than in.
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Health Conditions Discover Plan Connect. Medically reviewed by Debra Rose Wilson, Ph. How to calculate Don't know date of last period Irregular periods Doctor changes due date Ultrasound date Sources Overview.
How can I calculate my due date? What if I have irregular periods or long cycles? What does it mean if my doctor changes my due date? What is the ultrasound date, and why is it different from my due date? Article sources. Parenthood Pregnancy 1st Trimester. Placenta Delivery: What to Expect.
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