Accurate due date: How Realistic Is Your Due Date?

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How Realistic Is Your Due Date?


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A due date assumes pregnancy will last about 40 weeks, but it’s not meant to be a deadline for delivery.

Credit…Lucy Jones

By Alice Callahan

This story was originally published on Aug. 27, 2019 in NYT Parenting.

When I was 37 weeks pregnant with my first baby — three weeks before my due date — my obstetrician did a routine check of my cervix and noted that it was starting to dilate and shorten, signs that the wheels of the long birth process were beginning to turn. “You need to be prepared for labor to start any day,” I remember her saying, adding that I might have a baby within a week. I can’t remember if she emphasized the word might while reminding me that babies and birth were unpredictable, but if she did, I disregarded that and took her best guess as near-fact.

Ripe with the naivety and anticipation of a first pregnancy, I sprang into action. I installed the car seat, packed a hospital bag and scrambled to finish work projects. And then I waited. The days and weeks ticked by and my due date passed uneventfully. As my belly grew, so did my discomfort and impatience, until my daughter finally made her appearance — five days after her due date.

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Had I read the research on pregnancy length, instead of frantically preparing for birth and taking long walks to try to start labor (which hasn’t been proven to work), I might have appreciated how much pregnancies can vary, and how difficult it is to guess when a baby will be born. In fact, while I thought of my daughter as being late and overdue, like a library book racking up fines with each passing day, her arrival was well within the realm of normal.

A due date assumes pregnancy will last about 40 weeks, but it’s not meant to be a precise prediction or a deadline for delivery. A 2013 study of about 18,700 women in Australia, for instance, found that just 5 percent of births happened on their due dates.

“It’s an estimated time for the birth of your baby, with a big emphasis on estimated,” said Lisa Kane Low, Ph.D., a certified nurse midwife and a professor at the University of Michigan School of Nursing. Giving birth up to two weeks before and two weeks after are still considered normal, she said, though she noted that as inductions have become more common, it’s rare for pregnancies to last 42 weeks.

According to data from the Centers for Disease Control and Prevention, about 10 percent of the 3.8 million babies born in the United States in 2017 came preterm (before 37 weeks). Twenty-six percent were born in weeks 37 to 38; 57 percent in weeks 39 to 40; 6 percent in week 41; and less than 1 percent at 42 weeks or beyond. In 2017, 73 percent of babies were born before their due dates. Two decades earlier, in 1997, that figure was 57 percent. That difference is partly because inductions and cesarean births have become more common, but also because methods for estimating due dates have improved.

Still, predicting due dates is an imprecise science, largely because we rarely know exactly when pregnancy begins, which means there’s a lot of guesswork involved. Due dates are estimated by taking the first day of the last menstrual period and adding 280 days. But this assumes that we all have cycles lasting exactly 28 days (we don’t), that ovulation always happens on the 14th day (it doesn’t), and that we can accurately remember our last period (nope, not always). People who conceive with I.V.F. have more precise information about pregnancy timing, which is used to estimate their due dates, but even then, exact predictions are shaky.

With the exception of I.V.F. pregnancies, the American College of Obstetricians and Gynecologists says that fetal measurements taken during a first trimester ultrasound are the most accurate way to estimate a due date, especially for people with irregular menstrual cycles. Sometimes, initial due dates are revised after this ultrasound. An ultrasound done in the second or third trimester is less accurate for estimating due dates, because fetal growth becomes more variable as the pregnancy progresses.

For practical purposes, due dates allow expecting parents to plan for parental leave, child care, and the travel plans of family and friends who might come to help after the birth. But from a medical standpoint, they’re vital to tracking the progress of the pregnancy. “The tests we order, the counseling we give, the discussions we have are often based on the pregnancy time point in weeks, so having an accurate and unchanging due date is helpful,” said Dr. Christian Pettker, M.D., chief of obstetrics at Yale School of Medicine and one of the authors of ACOG’s guidelines on estimating due dates.

Even if you carefully tracked ovulation and know when your baby was conceived, your due date is still an estimate, because every pregnancy is different. That was demonstrated in a 2013 study in which researchers estimated the due dates of 125 women who were trying to conceive in the United States. They pinpointed the days they had ovulated by testing their urinary hormone levels and then followed their pregnancies. “What’s really cool is that even with this exact date, there was still five weeks of variability in length of pregnancy,” said Anne Marie Jukic, Ph.D., an epidemiologist at the National Institute of Environmental Health Sciences who led the study.

Observational studies have tried to tease out factors that might explain some of this variability. For example, one study of more than 40,000 women in London published in 2016 found that if a woman’s first baby came before or after her due date, her second baby tended to do the same, but not by as many days. Another study published in 2006 looked at more than 77,000 couples in Norway and concluded that gestational lengths might be inherited: meaning that the amount of time your child develops in the womb might be similar to the amount of time that you spent in your mother’s womb. And another study of about 119,000 women in Northern California found that those who were on their first pregnancies or who were obese were more likely to deliver at 40 weeks or beyond, while those with complications like high blood pressure or diabetes were more likely to deliver before their due dates.

But by their nature, observational studies can show only statistical correlations; they can’t demonstrate cause and effect. Additionally, much of this research — and thus our understanding of pregnancy length and factors influencing it — has been conducted in white populations and may miss important factors influencing pregnancy, and by extension, maternal and infant health. “We’re looking at research in incredibly homogenous populations that may not reflect the diversity in any one community,” said Dr. Amanda Williams, M.D., an ob-gyn and maternity director at Kaiser Permanente Oakland Medical Center in California.

Older studies in both the United States and Britain have found that a pregnancy’s length can vary with race and ethnicity, where white women tend to have longer pregnancies and are more likely to reach their due dates than black or Asian women. Black women in the United States are also at greater risk for preterm birth, which contributes to a higher rate of infant mortality among them. These outcomes are most likely caused at least in part by social inequality, the chronic stress of experiencing racial discrimination and, as reported in the journal Pediatrics in August 2019, disparities in NICU care.

Because of the data limitations on pregnancy length, Dr. Williams said she doesn’t dwell on the precision of due dates and the many factors that might nudge a baby to come a little earlier or later, explaining that they’re unlikely to be clinically significant for individual patients. “What I do tell them is that risk of preterm birth in African-American patients is much higher, so we’re going to take contractions much more seriously,” she said.

And when she’s weighing decisions like whether to induce labor, she’s not just thinking about the due date but other factors that are known to increase the risk to the baby as pregnancy continues, like diabetes or high blood pressure. “Medicine, especially pregnancy management, is as much an art as it is a science, and we have to individualize our care and take as much information as possible about that person and that pregnancy as we’re making decisions,” Dr. Williams said. “There are very few absolutes in obstetrics.”

I know I wished for more absolutes during the weeks of waiting for my daughter’s birth. I was used to having more control over my schedule, and it was hard for me to let that go. But in hindsight, that period was a fitting introduction to the unpredictability of babies and the patience necessary for parenting. Eight years later, my daughter still makes me wait daily: “Hang on, Mom, I’ll be there in a minute!”

Alice Callahan is a health and science journalist, a mom of two and the author of “The Science of Mom: A Research-Based Guide to Your Baby’s First Year.”

Pregnancy: Ways to Find Your Due Date – Health Information Library


There are several ways for your doctor or midwife to figure out how long you have been pregnant. They help you predict when you are likely to have your baby. This is called your due date. The due date is only an estimate of when your baby will be born. Most women deliver within 14 days of their due date.footnote 1

To find out how long you’ve been pregnant and when your baby is due, your doctor or midwife may:

  • Ask you about the first day of your last period.
  • Measure the size of your uterus.
  • Use ultrasound testing.

Last menstrual period

The most common way to calculate your due date is to start with the first day of your last menstrual period (LMP). Add 7 days, and then count backward 3 months. For example, if your last period started on March 20, you would add 7 days to get March 27. Then subtract 3 months to get a due date of December 27.

Another way to estimate your due date is to add 40 weeks to the first day of your last period.

  • Interactive Tool: What Is Your Due Date?

Size of uterus

Another way to find out your due date is by the size of your uterus. When you are about 12 weeks pregnant, your doctor or midwife can feel the top of the uterus (fundus) above your pelvis. After about 18 weeks, the distance between the pubic bone and the fundus (in centimeters) is likely to be about the same as the number of weeks since your last period. At 20 weeks, the fundus will be about as high as your belly button.

The size of the uterus is sometimes used to get a rough idea of how far along a pregnancy is. But it’s not an accurate way to predict the gestational age of the growing baby (fetus). That’s because there are many things that can make the fundus seem higher or lower than it really is. For example, the fetus may be in an odd position. Or you may have a uterine fibroid.

Ultrasound testing

If the first two methods can’t predict your due date, you may get an ultrasound. Ultrasound tests work well to find out how long you have been pregnant, especially if they are done before 20 weeks of pregnancy.footnote 2 Some doctors always do an ultrasound in early pregnancy.

During an ultrasound test, a small device called a transducer is moved back and forth over your belly. The transducer sends out sound waves that bounce off the fetus. The sound waves are converted by a computer into a picture of the fetus. The picture is displayed on a TV screen.



  1. Lund KJ, McManaman J (2008). Normal labor, delivery, newborn care, and puerperium. In RS Gibbs et al., eds., Danforth’s Obstetrics and Gynecology, 10th ed., pp. 23–42. Philadelphia: Lippincott Williams and Wilkins.
  2. American Academy of Pediatrics, American College of Obstetricians and Gynecologists (2007). Antepartum care. In Guidelines for Perinatal Care, 6th ed., pp. 83–137. Elk Grove Village, IL: American Academy of Pediatrics.


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new test for pregnant women will warn about childbirth in a few weeks

May 07, 2021

Olga Muraya

A clinical blood test that warns of the imminent onset of labor will allow the mother and doctors to properly prepare for this event.

Photo by Stanford Children’s Health.

Scientists from Stanford University have identified biomarkers in the blood of a pregnant woman, which indicate the preparation of her body for the upcoming birth.

The processes in the mother’s body that trigger childbirth have long remained unclear to scientists. Doctors are well versed in the stages of fetal formation, by which they determine the duration of pregnancy. However, when it comes to planning a birth, doctors still rely on ballpark calculations.

The expected date of delivery is counted from the end of the woman’s last menstrual cycle. Delivery approximately 40 weeks from now is considered term delivery. However, they can normally occur from the 37th to the 42nd week of pregnancy.

More accurate prediction of upcoming labor will help healthcare providers do a better job of protecting the health of mother and baby. For example, if it is determined in advance that a pregnant woman will soon begin preterm labor, doctors can prescribe steroid drugs for her to help open the baby’s lungs.

A new study published in Science Translational Medicine describes for the first time a method for predicting the onset of labor based on immune and other biological signals in a pregnant woman’s blood.

Now scientists finally have an understanding of how a woman’s body prepares for such an amazing event in every sense as childbirth.

«We have found a transition from an advanced pregnancy to the prenatal phase, which begins two to four weeks before delivery. We have found a new way to use maternal blood to predict when a mother will go into labor,» explains study lead author Dr. Ina Stelzer (Ina Stelzer of the Stanford University School of Medicine.

The research team followed the last 100 days of pregnancy in 63 women. All of them gave blood samples for research 2-3 times before delivery. All women gave birth naturally, that is, none of the births were artificially induced.

All blood samples were analyzed for more than 7,000 immune parameters. Each of them was compared with the number of days before the onset of childbirth. Thus, using mathematical modeling, scientists were able to identify specific indicators in the blood that increase shortly before the onset of labor.

As a woman’s body entered the prenatal phase, her blood tests began to show elevated levels of the hormones progesterone and cortisol. This confirms the results of previous studies on the physiology of late pregnancy.

Also, the concentration of vascular growth factors in the blood of pregnant women decreased, which is most likely associated with the process of separation of the placenta from the uterus. At the same time, the number of blood clotting factors increased, which may indicate the preparation of the body to prevent excessive blood loss during childbirth.

Researchers have also found that a woman’s immune system is fine-tuned during the prenatal period. The mathematical model revealed that the most accurate determinant of imminent delivery is the appearance in the blood of the immune protein IL-1R4, which suppresses the inflammatory marker interleukin-33.

The hypothesis of scientists was that childbirth is an inflammatory reaction of the body. The results partially confirmed this assumption, but at the same time, the researchers saw that the signs of inflammation weakened before childbirth. The authors of the work believe that this indicates the preparation of the mother’s immune system for the postpartum period, in which there is an active recovery process.

In the future, the scientists plan to confirm their findings by increasing the number of participants in the study. The authors of the work are trying to make their test more accurate, because so far it allows you to predict childbirth only within two weeks. Researchers plan to create a qualitative test in two to three years, which can be used by doctors everywhere to determine early labor in healthy pregnancies.

Earlier we wrote that scientists have identified possible signs of premature birth in the blood of pregnant women. We also reported that the immune system is engaged in real «molecular diplomacy» in the body of a pregnant woman.

More news from the world of science and medicine can be found in the sections « Science » and «Medicine» on the media platform « Watching «.

blood analysis

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  • Why in the UK they create children with the DNA of three parents
  • Children of three parents were born in the UK
  • Pregnancy hormones help quit smoking?
  • First evaluated the safety of creating children from three parents
  • Coffee during pregnancy: is it still possible or not?
  • Depression after childbirth is associated with changes in the immune system

Which date of birth is the most accurate

Articles › Ultrasound › Obstetric term is longer than the term according to ultrasound

Add 7 days to the date of the first day of the last menstruation, subtract 3 months and add one year (plus 7 days, minus 3 months). Thus, we get the estimated date of birth, that is, exactly 40 weeks. It works like this: For example, the date of the first day of the last menstruation is 02/10/2021.

  1. What is the most accurate due date
  2. Which gestational age is more accurate by ultrasound or by menstruation
  3. Is it possible to determine the exact gestational age by ultrasound
  4. What is the difference between an obstetric term and a gynecological term
  5. Why do ultrasounds put a period longer than
  6. How to understand that childbirth is close
  7. Why gynecologists put a term longer than
  8. What is the gestational age or rather obstetric or ultrasound
  9. How long is the correct obstetric or ultrasound
  10. Why the obstetric period is longer than according to ultrasound
  11. How many weeks is a boy pregnant
  12. How to set the preliminary date of delivery
  13. When to give birth
  14. What time is indicated on ultrasound
  15. How to understand that the birth will be before the due date
  16. When they give birth prematurely
  17. What is the latest due date
  18. Why do gynecologists calculate the gestational age of
  19. How many weeks PDR
  20. When will I give birth
  21. Who puts an obstetric or fetal term
  22. How to correctly count the beginning of pregnancy
  23. When labor usually begins
  24. How to bring the due date closer
  25. What is the minimum period between births

What is the most accurate due date

Since the exact time of fertilization cannot be determined, the date of onset of pregnancy is traditionally considered the first day of the last menstruation. The easiest way to calculate the EDA is to add 9months and 7 days to the first day of the last menstrual period.

Which gestational age is more accurate according to ultrasound or monthly

Ultrasound is the most accurate way to diagnose pregnancy. With the help of ultrasound with a transvaginal sensor, it is possible to determine the presence of a fetal egg in the uterus already 1-2 weeks after conception (3-4 obstetric weeks), but it is possible to detect fetal heartbeats only for a period of 5-6 obstetric weeks.

Is it possible to determine the exact gestational age by ultrasound

If ultrasound is done for up to 7 weeks, then the date of conception can be determined most accurately, with an error of 2-3 days. During this period, the embryo develops proportionally and its size is approximately the same in all women.

What is the difference between the obstetric term and the gynecological term

The fact is that the expectant mother tries to calculate the gestational age in real, so-called embryonic weeks. A gynecologist — in obstetrics. The difference between them is approximately 14 days. Obstetric weeks are considered a more accurate method of measuring pregnancy.

Why is the term more than

When calculating the gestational age by menstruation and by ultrasound, there may be discrepancies. With ultrasound, the size of the embryo may be greater than the expected period for menstruation. And if the menstruation was not very regular before the onset, then the gestational age may not correspond to the first day of the last menstruation.

How to understand that the birth is close

Harbingers — the birth is coming soon!:

  • False contractions They may appear after the 38th week of pregnancy.
  • Abdominal prolapse
  • Mucus plug discharge
  • Weight loss
  • Change chair
  • Change of mood

Why do gynecologists put a term longer than

This is because, according to medical criteria, pregnancy lasts 10 months. That is, this period is counted from the first day of the last menstruation, which is considered the estimated date of conception. Thus, it turns out that ovulation was 2 weeks ago, and the obstetrician-gynecologist will write down the term for 4 weeks.

Which gestational age is more accurate than obstetric or ultrasound

Embryonic gestation is always shorter than obstetric by 12-16 days. Computational methods are not an alternative to ultrasound diagnostics, but only complement it and help determine if there are violations of intrauterine development of the fetus. Ultrasound remains the most accurate.

What is the correct obstetric or ultrasound

It should be remembered that all doctors use only the obstetric term, all test results, fetal size, timing of examinations, maternity leave and the term of delivery are calculated taking into account only the obstetric term of pregnancy.

Why the obstetric period is longer than according to ultrasound

Because, during pregnancy, they distinguish:

The obstetric period of pregnancy is counted by gynecologists from the first day of the last menstruation, due to a simpler calculation. Embryonic is the real gestational age, but it cannot be determined, neither by the doctor nor by the woman.

How many weeks a boy is pregnant

Typically, only 5% of women give birth on their due date, writes Tracey S. Johnson, MD, M.D. How many weeks does a boy pregnancy last? The approximate term for carrying a child, regardless of gender, is 40 weeks (280 days), writes WebMD.

How to calculate the due date

How to calculate the EDD? To calculate the EDD (estimated date of birth), the Negel formula is used: To do this, add 40 weeks to the first day of the last menstruation, or count 3 months from the first day of the last menstruation and add 7 days to the resulting number.

When to give birth

In most cases, the birth of a child occurs in the interval plus a few days — minus two weeks from the appointed date. The term is determined as follows — add 40 weeks (280 days) to the first day of the last menstruation.

What is the term indicated for ultrasound

Ultrasound 1 screening — a period of 11-14 weeks; Ultrasound 2 screening — period 18-21 weeks; Ultrasound 3 screening — a period of 30-34 weeks.

How to understand that the birth will be earlier than the PDR

Symptoms of premature birth:

  • pulling pains in the lower abdomen, resembling primary contractions;
  • feeling of pressure in the genitals of a pregnant woman;
  • high fetal activity;
  • discharge from the genitals, sometimes mixed with blood;
  • frequent urge to urinate and defecate.

When is a premature birth

What is preterm birth? Pre-term births up to three weeks before the expected date of delivery: less than 37 weeks of gestation, while the full gestation is 40 weeks. Premature birth is also called preterm birth.

What is the latest due date

In 2016, Wang Shi from the Chinese province got into the Guinness Book of Records due to the fact that her pregnancy lasted a whole 17 months! The girl became pregnant in February 2015, and according to the doctors, the child was supposed to be born in November.

Why gynecologists calculate the gestational age

To determine the exact date of the baby’s birth, gynecologists use a table according to which normal birth occurs at a period of 40 weeks and 9 days from the first day of the last menstruation. Therefore, the notorious 9 months of pregnancy is actually 10 months, if you count the obstetric weeks.

How many weeks EDD

Obstetric EDD is calculated from the date of the last menstrual period. To do this, use the Negele formula: three months are taken from the first day of the last menstruation and 7 days are added, which turns out to be exactly 40 weeks. This is the most common, although not the most accurate option.

When will I give birth

First of all, you need to accurately determine the first day of the last menstruation. Then subtract three months and add 7 days to the resulting day. We’ll get the due date.

Who sets the obstetric or fetal period

The ultrasound determines the embryonic period according to the size of the fetus, and the obstetric period is set by a gynecologist.

How to correctly calculate the beginning of pregnancy

The easiest way to determine the duration of pregnancy is to start from the date of the last menstruation. After a successful conception, the start of the next menstruation falls on the 4th week of pregnancy. This technique implies that a fertilized egg begins to divide even before ovulation.

When labor usually begins

In practice, some begin before 20 weeks of gestation, others only by 30 weeks, and some only experience contractions-precursors that begin a couple of weeks before the birth. There are women who do not feel any training tension of the muscles of the uterus, and this is also normal.

How to bring the date of birth closer

What provokes childbirth:

  • Sex.
  • Walks.
  • Warm bath.
  • Laxative (castor oil).
  • Acupressure points massage, aromatherapy, herbal teas, meditation — all these procedures can also help, help to relax and improve blood circulation.

What is the minimum period between births

WHO recommends an interval between pregnancies of 18-24 months. Pregnancy less than 12 months after the previous one carries risks for women at any age.

By alexxlab

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