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Determining Due Date and Doing Routine Diagnostic Tests and Procedures

Determining Due Date and Doing Routine Diagnostic Tests and Procedures
How important is the due date

Most doctors refer to the due date as EDD or Estimated Date of Delivery. And EDD are just that, estimated dates, not exact dates. What this means is that due dates are not sacred, even if your periods are regular. But due dates are helpful for the expectant mother to prepare herself for childbirth and also to plan for the new arrival in the family. An estimated due date can certainly help in planning for all of the following:

  • Physical exercise for the mother (muscle-toning for childbirth);

  • Mental preparation (possible change in lifestyle);

  • Financial adjustments (babies cost money, even before they reach college!);

  • Maternity leave planning (giving your company a notice period); and

  • Most fun of all - baby shopping.
But very few women actually deliver on their due date! Less than 50% of expectant mothers deliver on their expected due date. If your periods come at a regular interval of 28 days, you are more likely to deliver close to your estimated due date. If your cycles are longer than 28 days, you are more likely to deliver later than your EDD, and if they are shorter, earlier. However, if you have irregular cycles, then this dating system may not work; a sonogram taken later in the pregnancy may be a better indicator.

How to Determine your Due Date

A pregnancy generally lasts for 40 weeks from first day of your last menstrual period, or 38 weeks from conception. If you know the date of your last menstrual period, or the date of conception (some women do!), and your cycles are regular, you should be able to determine your due date sitting at home.

Based on LMP:

If your cycles are regular and 28 days long, then you can determine your due date by adding 9 months and 7 days to the first day of your last menstrual period (LMP). For example, if your LMP began on September 5, then your expected due date (EDD) would be June 12 of the following year.

Based on date of Conception:

If your cycles are 28 days long and you know your date of conception, you can determine your due date by first subtracting 7 days, and then adding 9 months. However, this method will probably not work for most women unless they have had intercourse only once during the fertile period of that cycle and remember the date.

Abdominal Examination:

The doctor will examine your abdomen at each of your regular monthly visits. One of the things that the doctor will be checking for is the size and growth of the baby. Although this method cannot pinpoint a specific date for delivery, it can certainly tell the doctor if the due date estimated by either the LMP or the date of conception is in the correct range or not. For instance, sometimes the LMP may predict the due date as April 15th. But based on an abdominal examination the doctor may feel that the baby is very big (growing too fast), he/she could revise the due date and move it up by 5-10 days (or even more) to perhaps April 8th.

Ultrasound Scan:
One of the reasons that the doctor asks for frequent ultrasound scans, especially in the last few months, is that they can determine the development of the baby and provide a more accurate due date. This method is superior to other methods because the doctor is able to examine and measure each of the baby's organs on ultrasound screen. These measurements can then help in determining the size of the baby, i.e. stage of fetal development, and consequently the due date. It is possible that each of your ultrasound scans gives a different due date. The date computed from your last ultrasound scan is likely to be the most accurate. Again, remember that these dates are simply estimates, and therefore are only indicative of the actual delivery date.

Can different methods give different due dates?

Yes. Each method will most probably give a different due date, but most of the time, they will vary only by a couple of days. However, if the woman has an irregular menstrual cycle, or her cycles are very long or very short, then the LMP method will not be accurate. In this case, an ultrasound scan will be most accurate.

Can I also plan to have my baby on a particular day?

Yes, you can try to plan your baby for a particular day. However, remember, your menstrual cycles and ovulation times would have to cooperate! If you want to have your baby on a particular day, you can subtract 38 weeks from the planned due date. If it is easier to work in months and days, you should first add 7 days from your planned due date, and then subtract 9 months.

What diseases are regularly tested for even during a normal pregnancy

Some of the conditions routinely checked for in most normal pregnancies are Aids, Hepatitis B, Down's Syndrome, Neural Tube Defects, Rh Incompatibility and Rubella. The doctor can test for all these condition with blood samples, albeit taken at different times in the pregnancy.

Your baby has a higher chance of having congenital disease if he falls in any of the three reasons given below. Note that there could be many other reasons for congenital diseases also.
  • Congenital diseases in the previous baby

  • History of congenital anomalies in the family which are likely to be repeated

  • Maternal age is above 35, then there is a higher chance of the baby getting Down's syndrome
Can birth defects be detected with regular blood tests?

Studies have shown that 90 % of birth defects cannot be detected through prenatal blood tests. In order to isolate the remaining 10 % of the identifiable birth defects, there are four main types of tests: amniocentesis, chorionic villi sampling, certain blood tests like the Alphafeto-Protein (AFP) test, and ultrasound scans.

There are various conditions or diseases that a doctor can identify by conducting different diagnostic tests during the pregnancy. These tests can indicate the possibility of birth defects in the foetus or conditions in the mother's health that may lead to problem pregnancies. Once any potential problem has been identified, appropriate steps can be taken to deal with it.

Diagnostic testPossible Findings
Regular blood testConfirmation of pregnancy (hCG hormone), blood group and Rh- disease, heamoglobin levels, platelet count
AIDS blood testAIDS
Hepatitis B blood test Hepatitis B
Rubella blood testRubella (German Measles)
Alphafeto-protein (AFP) blood testNeural Tube Defects (NTD)
Urine TestConfirmation of pregnancy (hCG hormone), sugar, albumin, bacteria
Sonogram (Ultrasound Scan)Genetic deformities (including Down's syndrome)
AmniocentesisSpina bifida (Open Spine), Anencephaly (Failure of brain formation), Down's syndrome
Chorionic Villi Sampling (CVS) Tay-Sachs, sickle-cell anaemia, Down's syndrome, thalassaemia

List of Common Tests

Alpha Feto-protein (AFP)

This is a blood test that measures the amount of alpha-foeto protein (AFP) in the mother's blood and the amniotic fluid. It is generally performed in the 15th to 18th week of pregnancy. High levels of AFP could indicate that the baby has a neural tube defect (NTD). This defect manifests itself as spina bifida (deformity of the spinal column) or anencephaly (the absence of all or part of the brain). Abnormally low levels could be a sign of increased risk of Down's syndrome or other chromosomal defect. 95% of the time there is no past family history of these conditions.

The drawback of this test is that abnormally high or low levels of AFP in the blood do not necessarily indicate that your baby will have a birth defect. Statistics show that while only 1 to 2 women in a 1000 have a foetus with a neural tube defect (NTD), about 50 of these women will demonstrate high levels of AFP in their blood when tested. Thus, to make a positive diagnosis, further testing such as amniocentesis and a sonogram becomes necessary. An elevated level of AFP in the amniotic fluid is associated with a 90 % chance that the foetus has NTD.


This test is helpful in diagnosing birth defects such as neural tube defect (NTD), including anencephaly (failure of brain formation) and spina bifida (open spine). A small sample of the fluid in which the baby lies is drawn off through a needle. The test measures the amount of fetal protein both in the mother's blood as well as in the amniotic fluid surrounding the baby.

An amniocentesis is usually advised for women in the following high risk category for birth defects:

* Women over 35 (since the risk of Down's syndrome is very high)

* Women with three or more miscarriages

* If either parent or sibling has a known chromosomal abnormality or a hereditary disease

Chorionic Villi Sampling (CVS)

Chorionic Villi Sampling (CVS) is a test that gives a complete picture of the genetic makeup of the developing foetus. This procedure involves taking a sample of the chorion, a membrane surrounding the foetus, either via the vagina and cervix, or through a needle inserted in the abdominal wall. Since the sample is taken directly from the foetus, its examination can help in identifying any defects in the baby.

CVS is useful in detecting disorders like Tay-Sachs, sickle-cell anaemia, most types of cystic fibrosis, the thalassaemias, and Down's syndrome.

The advantage of CVS over amniocentesis is that it can be performed as early as the tenth week of pregnancy and gives results within one or two weeks. Thus, doctors can identify genetic defects earlier on in the pregnancy.

Amniocentesis can only be conducted around the 16th week of pregnancy. The results of CVS are also available earlier and sometimes, CVS can be performed when there is not enough amniotic fluid.

Hepatitis B Test

Unlike hepatitis A, this strain can be passed on to the foetus during childbirth. Nine out of ten infected babies, if left untreated, become chronic carriers and are at a risk of developing a more serious liver disease.

This disease is generally tested in the fifth month of pregnancy. If the test results indicate that you do have hepatitis B, it is possible for your doctors to treat your baby right at birth, so that future complications are prevented.

Sonography (ultrasound)

An ultrasound scan gives a picture of the baby in the mother's womb without using X-rays. It is used to check the baby's development. By using sound waves, birth defects can be identified by the 16-18 week of gestations and sometimes even earlier.

Most doctors, these days, advise 3-4 routine sonography tests: first test around the 6th week of pregnancy, second test around the 19th week, third one in the 7th month; and the fourth test in the 37th week. In addition to birth defects, it also helps in identifying twins (or multiple births), and position of the baby and placenta closer to the due date.

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