What is the average gestational age at delivery

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.

Some of the researchers used open-ended stillbirth rates, and some of them used actual stillbirth rates. All of the researchers found a relative increase in the risk of stillbirth as pregnancy advanced. To get an accurate picture of stillbirth in people who go past their due date, it would be best to look at studies that took place in more recent times. To see all of the other studies, click to view the entire table here. All 3 of these studies used the actual stillbirth rate—not the open-ended stillbirth rate.

Two studies used ultrasound to calculate gestational age, and one study used the LMP. The largest meta-analysis to date on risks of stillbirth and newborn death at each week of term pregnancies was published in Muglu et al.

The researchers included 13 studies 15 million pregnancies, nearly 18, stillbirths. The risk of stillbirth per 1, was 0. Based on their data, Muglu et al. To experience one additional stillbirth, there would need to be at least 2, people waiting for labor for one more week starting at 39 weeks. At 40 weeks, 1, people would have to wait for labor for one more week to experience one additional stillbirth.

At 41 and 42 weeks, only and people, respectively, would have to wait for labor for one more week to experience one additional stillbirth. The researchers also found evidence that health care systems are failing Black mothers and babies—an alarming but common theme in health care research.

Black mothers were 1. When they looked only at low-risk pregnancies, the risk of stillbirth was 0. Low-risk pregnancy was defined as pregnancies with a single baby, no congenital abnormalities, and no medical conditions in the mother. There was no additional risk of newborn death when giving birth between 38 and 41 weeks, but the risk of newborn death did increase beyond 41 weeks.

Other factors that do not necessarily cause stillbirth but may increase the risk of stillbirth, in general, include:. Of course, parents can still experience the stillbirth of a child even when none of these risk factors are present. To read more about theories of unexplained stillbirth, read this article here. However, up until recently, there was no research on this topic. In , researchers published the first study looking at biological markers of aging in placentas.

In this study, researchers in Australia collected placentas from 34 people who gave birth between weeks of pregnancy, 28 people who gave birth between weeks, and 4 people who experienced stillbirths between 32 and 41 weeks Maiti et al. Five or more tissue samples were removed from each placenta, and the samples were analyzed using a variety of biochemical tests.

Overall, the analysis of the placentas from the week pregnancies and from the stillbirths showed increased signs of aging, with decreased ability to transport nutrients to the baby and waste products away from the baby, compared to the placentas from the earlier term births.

The rate of placental aging varied in different pregnancies, and the authors stated that not all of the week placentas showed signs of aging. We reached out to the authors to find out more, and they told us that one-third of the week placentas showed increased signs of aging compared to the week placentas.

This means that two-thirds of the week placentas did not show signs of aging. You can watch a minute video describing the findings of this emerging research here. I would also like to thank my expert reviewers for an earlier version of this article—Shannon J.

Join others who also want to help bring evidence-based care to their local community. Rebecca Dekker Don't miss an episode!

Don't miss an episode! Figure 1 illustrates a combination of categories of preterm birth, low birth weight, and small for gestational age infants. A combination of preterm, low birth weight, and small for gestational age infants. Thus to define IUGR or grow restriction, a birth weight for gestational age standard with the tenth percentile birth weight defined is needed [ 6 , 7 ]. Important pregnancy outcomes include neonatal mortality, stillbirth, long-term neurologic problems, and maternal mortality [ 3 ].

Research conducted in this area indicates that many of these outcomes are associated with length of gestation or gestational age of the infant at birth. In the United States and other developed countries, pregnancy outcomes are much better than those in many developing countries, where the adverse outcomes mentioned above are increased 10—fold as compared to US rates [ 3 ].

However, adverse pregnancy outcomes are generally more common in the United States than other developed countries [ 5 , 8 ]. Low infant birth weight, either due to preterm birth or intrauterine growth restriction, is attributed to much of the infant mortality, morbidity, and increased cost of perinatal care.

Wide disparity exists in both preterm and growth restrictions among different population groups. Poor blacks, for example, have twice the preterm birth rate and higher growth restriction than do most women [ 5 ]. The infant mortality from to for non-Hispanic blacks decreased from For non-Hispanic white infants, the rate decreased monotonically from 5. Further analyses indicate that black infants are nearly 2. Riddell et al. Also, black infants experience nearly four times as many deaths related to short gestation and low birth weight, making it the leading cause of infant deaths among black infants during the first year of life [ 8 ].

See Figure 2. Thus, these results confirm that gestational age at birth is a significant factor which affects major factors resulting in poor pregnancy outcomes such as infant mortality and morbidity.

The risk of adverse consequences declines with increasing gestational age [ 9 ]. Infant mortality rate of black versus white infants. In this chapter, pregnancy outcomes and its relationship to gestational age will be limited to pregnancy outcomes associated with preterm birth, race and ethnicity, low birth weight, stillbirth, small for gestational age fetal growth restriction , and certain chronic diseases. As noted before, preterm birth is defined as infants born before completing gestational age of 37 weeks.

In , an estimated Preterm birth also affects affluent countries, for example, the United States has high rates and is 1 of 10 developed countries with the highest number of preterm births [ 10 ]. Since decreasing gestational age is associated with increasing mortality, disability, and cost due to intensity of neonatal care, these subdivisions are important [ 7 ].

As stated before, the risk of adverse consequences declines with increasing gestational age [ 9 ]. There are a variety of causes of preterm birth which can be broadly classified into 1 provider initiated preterm birth induction of labor or elective cesarean section before 37 weeks of gestation for maternal or fetal indications, 2 spontaneous preterm labor with intact membranes, and 3 preterm rupture of the membranes PPROM , irrespective of whether delivery is vaginal or by cesarean section [ 9 ].

Births that follow spontaneous labor and PPROM are together referred to as spontaneous preterm births. The contributions of the causes of preterm birth to all preterm births differ by ethnic groups.

PPROM most commonly is the cause of preterm birth in black women, but spontaneous preterm birth is most commonly caused by preterm labor in white women [ 11 ]. Obstetric intervention or iatrogenic preterm birth explains much of the increase seen in preterm births [ 12 , 13 ]. Also, prior spontaneous preterm delivery is strongly associated with recurrence in the current pregnancy. An early prior spontaneous preterm delivery is a better predictor of recurrence and is most strongly associated with subsequent early spontaneous preterm delivery [ 14 ].

Ananth et al. Also a considerable increase is associated with multiple births that occur due to the use of various assisted reproductive techniques. Demographic and socioeconomic factors associated with increased risk of preterm birth include ethnicity, the presence of indicators of low socioeconomic status, and extreme maternal age among other factors [ 9 ].

A country-based study conducted in Italy showed an association between preterm birth and certain maternal outcomes as BMI, employment, previous abortions, previous preterm delivery, and previous cesarean section [ 16 ]. Some researchers believe preterm labor to be a syndrome initiated by multiple mechanisms, including infection or inflammation, utero-placental ischemia or hemorrhage, stress, and other immunologically mediated processes [ 11 , 17 ]. A more concise mechanism is difficult to establish in most cases; therefore factors linked with preterm birth have been analyzed to explain preterm labor.

Since many of the risk factors result in systemic inflammation, increasing infection or inflammation pathway might explain some of the variance in increase in preterm births associated with multiple risk factors [ 18 ].

Maternal BMI is an important risk factor for preterm birth and is of public health importance independently. Others support an increase in provider initiated preterm birth with increasing BMI [ 22 , 23 ]. Cole-Lewis et al. However, only about four per cent of women actually give birth on their EDC. There are many online pregnancy calculators see Baby due date calculator that can tell you when your baby is due, if you type in the date of the first day of your last period.

A simple method to calculate the due date is to add seven days to the date of the first day of your last period, then add nine months. For example, if the first day of your last period was 1 February, add seven days 8 February then add nine months, for a due date of 8 November. Some clues to the length of gestation include:. The general procedure for a pregnancy ultrasound includes:.

A baby born prior to week 37 is considered premature. The closer to term estimated date of confinement, or EDC the baby is born, the higher its chances of survival - after 34 weeks gestation with good paediatric care almost all babies will survive.

Premature babies are often afflicted by various health problems, caused by immature internal organs. Respiratory difficulties and an increased susceptibility to infection are common. Often there is no known cause for a premature labour; however, some of the maternal risk factors may include:. Around five out of every babies will be overdue, or more than 42 weeks gestation.

If you have gone one week past your due date without any signs of impending labour, your doctor will want to closely monitor your condition. Tests include:. The placenta starts to deteriorate after 38 weeks or so, which means an overdue baby may not get enough oxygen.

An overdue baby could also grow too large for vaginal delivery. Generally, an overdue baby will be induced once it is two weeks past its expected date.

Some of the methods of induction include:. This page has been produced in consultation with and approved by:. In Victoria, you can have two types of abortion: surgical and medication. Both types are safe and reliable. You can have a medication abortion up to nine weeks of pregnancy.