Can blood be drawn for prenatal DNA testing before 6 weeks of pregnancy?

Rewritten on: January 8, 2025

For prenatal DNA testing, fetal DNA becomes detectable in the mother's blood starting at 6 weeks of pregnancy, but a quantity sufficient for testing is only obtained from 7 weeks onward. This article explains in detail the relationship between blood draw timing and the amount of fetal DNA.

When can blood be drawn for prenatal DNA testing? Why testing is not possible with blood from before 6 weeks of pregnancy

For anyone considering prenatal DNA testing, one of the biggest questions is likely "from when can blood be drawn?" To give the conclusion up front, prenatal DNA testing cannot be performed using blood drawn before 6 weeks of pregnancy. This is because fetal-derived DNA is not yet present in a sufficient quantity in the mother's blood.

In prenatal testing, the fetal DNA contained in the blood of a pregnant woman is compared with a man's DNA to determine whether that man is the biological father of the fetus. Generally, fetal DNA begins to flow into the mother's blood from around week 6 of pregnancy, and the amount of fetal DNA in the blood increases in proportion to the length of pregnancy, becoming roughly constant from week 14 onward(1).

This article explains in detail, based on scientific evidence, the appropriate timing for a blood draw in prenatal DNA testing, taking into account the developmental process of the fetus in early pregnancy and the behavior of cell-free fetal DNA (cffDNA) in maternal blood.

Why does fetal DNA start flowing into the mother's blood from week 6 of pregnancy?

Here's a question for you. Why does fetal DNA start flowing into the mother's blood specifically at week 6 of pregnancy? You might think it should start flowing even earlier.

To answer this question, we need to understand how the fetus develops during pregnancy. Let's organize the key steps of early pregnancy in chronological order below.

How pregnancy weeks are counted and the early developmental process

The World Health Organization (WHO) defines the first day of the last menstrual period as the first day of pregnancy, i.e., "week 0, day 0"(2). This definition is used worldwide, and the same standard is adopted by obstetric and gynecological clinics in Japan.

About two weeks after "week 0, day 0," the egg that will eventually become a fertilized egg and grow into a fetus is ovulated. In other words, during weeks 0–2 of pregnancy, fertilization has not actually occurred yet. Many people find this surprising, but it's a very important point in understanding how gestational weeks are counted.

Since it takes 6–12 days from fertilization to implantation, pregnancy is not actually established until weeks 3–4. Once implantation occurs and pregnancy is established, the placenta and umbilical cord begin to form, and the fertilized egg rapidly develops into an "embryo," becoming a "fetus" at week 9_{Note 1}.

1. Weeks 0–2: From the last menstrual period to ovulation. Fertilization has not yet occurred
2. Weeks 2–3: Ovulation and fertilization occur, and the fertilized egg travels through the fallopian tube to the uterus
3. Weeks 3–4: Implantation is completed and pregnancy is established. Placental formation begins
4. Weeks 4–5: The period when a typical home pregnancy test begins to show a positive result
5. Week 6: The fetal heartbeat can be confirmed by ultrasound, and fetal DNA becomes detectable in maternal blood
6. From week 7 onward: A level of fetal DNA sufficient for use in DNA testing is secured

Incidentally, a typical home pregnancy test is said to show a positive result around weeks 4–5 of pregnancy, and the fetal heartbeat can be confirmed by ultrasound around week 6. Confirming the heartbeat is an important indicator that the fetus is developing normally, and it also serves as a reference point when determining the timing for a blood draw for prenatal DNA testing.

At 3–4 weeks of pregnancy, the amount is too small to detect

Fetal DNA is present in the mother's blood because fetal-derived cells flow into the mother's bloodstream through the placenta_{Note 2}. These fetal-derived DNA fragments circulating in maternal blood are called "cell-free fetal DNA (cffDNA)," and they are released into the maternal circulation mainly during apoptosis (natural cell death) of the placental trophoblast (nutrient membrane cells)(3).

Even at weeks 3–4 of pregnancy, when placental and umbilical cord formation has begun, fetal-derived DNA is likely already flowing into the mother's blood, but in such a minute amount that it cannot be detected. This is because, in early pregnancy, the placenta is not yet sufficiently developed, and the amount of fetal DNA fragments released into maternal blood is extremely small.

The relationship between the amount of fetal DNA in maternal blood and gestational week

As gestational weeks progress, the amount of fetal DNA in maternal blood increases, finally becoming detectable at week 6, and reaching a level usable for DNA testing by week 7. According to research, the proportion of cffDNA in maternal plasma (the fetal fraction) is around 3–10% in early pregnancy (the first trimester), and this increases as gestation progresses(4). From week 14 onward it stabilizes at a roughly constant proportion, and in some cases it can exceed 20% just before delivery.

However, the amount of fetal DNA in maternal blood varies greatly between individuals, and even among people at the same gestational week, whether a sufficient amount is present to determine a biological relationship cannot be known until the test is actually performed. Factors that can affect the amount of fetal DNA include the following.

• Maternal BMI (body mass index): the higher the BMI, the more maternal-derived DNA there tends to be, which relatively lowers the proportion of fetal DNA
• Placental size and condition: the amount of cffDNA released varies depending on how the placenta has developed
• Errors in gestational week calculation: irregular menstrual cycles, for example, can cause a discrepancy between the actual gestational week and the calculated one
• Multiple pregnancy: interpretation of fetal DNA quantity may differ in the case of twins or more
• Ectopic pregnancy or fetal condition: if the pregnancy is ectopic, or if the fetus had already passed away at the time of the blood draw, fetal DNA cannot be detected in the mother's blood

Risks and precautions for drawing blood before 7 weeks of pregnancy

On rare occasions, some clients submit blood before 7 weeks of pregnancy, but this carries a high risk of requiring a retest because fetal DNA cannot be extracted, and it also falls outside the scope of our free retesting and money-back guarantee. Prenatal DNA testing is by no means inexpensive, so to obtain an accurate result the first time, it is important to always have blood drawn from week 7 of pregnancy onward.

DNA testing becomes possible from 7 weeks of pregnancy onward

In summary, pregnancy is established at weeks 3–4, the amount of fetal DNA in maternal blood gradually increases, and DNA testing becomes possible from week 7 of pregnancy onward.

We recommend having an ultrasound scan at an obstetrics and gynecology clinic beforehand to confirm a normal pregnancy, and then having blood drawn once you have entered week 7 of pregnancy. Confirming a normal pregnancy is also an essential step for ruling out abnormalities such as an ectopic pregnancy.

Also, calculating gestational age based on menstrual cycle, last menstrual period, or ovulation date is not accurate, so please always confirm the gestational week with an obstetrics and gynecology clinic. Measuring fetal size via ultrasound at an obstetrics and gynecology clinic is the most accurate method for determining gestational age(5).

»"Can the father be identified from the menstrual cycle or ovulation date?"

Points to check before a blood draw for prenatal DNA testing

To ensure the success of prenatal DNA testing, please be sure to check the following points before the blood draw.

• Have an ultrasound scan at an obstetrics and gynecology clinic to confirm a normal intrauterine pregnancy
• It is preferable that the fetal heartbeat has been confirmed by ultrasound
• Do not calculate the gestational week yourself — have it confirmed by a doctor at an obstetrics and gynecology clinic
• Have blood drawn from week 7 of pregnancy onward (ideally from week 8 onward)
• If you have any questions, consult with seeDNA's specialist staff in advance

The scientific background of cffDNA (cell-free fetal DNA)

Fetal-derived DNA fragments (cffDNA) present in maternal blood were first discovered in 1997 by a research group led by Dennis Lo and colleagues(6). Since this groundbreaking discovery, non-invasive prenatal testing technologies using cffDNA have advanced dramatically, and are now applied to a variety of tests, including NIPT (Non-Invasive Prenatal Testing) and prenatal DNA paternity testing.

cffDNA is released mainly from the placental trophoblast cells and makes up a certain proportion of the total cell-free DNA (cfDNA) in maternal plasma. A characteristic feature of cffDNA is that it consists of very short fragments (approximately 150 base pairs), which tend to be shorter than maternal-derived cfDNA(7). This characteristic makes it possible to distinguish maternal DNA from fetal DNA and to obtain the genetic information needed for DNA testing.

It has also been reported that cffDNA disappears rapidly from maternal blood after delivery, becoming almost undetectable within about 2 hours(4). This is because cffDNA is quickly broken down and eliminated by the liver and kidneys, so there is no need to worry about fetal DNA from a previous pregnancy affecting the next test.

Summary of changes in fetal DNA quantity by gestational week

Below is a concise summary of the main changes and the state of fetal DNA in early pregnancy.

*From week 7 of pregnancy onward, DNA testing becomes possible, and from week 14 onward the amount of fetal DNA becomes roughly stable.

Note 1: On our website, we consistently use the term "fetus" even for periods before week 9 of pregnancy.

Prenatal DNA testing from the DNA testing specialist "seeDNA"

Identify the father of the baby in the womb
from week 6 of pregnancy

Frequently Asked Questions

Q1. From what week of pregnancy can blood be drawn for prenatal DNA testing?

A. The amount of fetal DNA in maternal blood reaches a level sufficient for testing from week 7 of pregnancy onward. Fetal DNA becomes detectable at week 6, but we recommend a blood draw from week 7 onward to ensure testing accuracy. Please have an ultrasound scan at an obstetrics and gynecology clinic beforehand to confirm a normal pregnancy and the gestational week before having blood drawn.

Q2. Why can't testing be done with blood from before 6 weeks of pregnancy?

A. Fetal DNA flows into the mother's blood through the placenta, but in early pregnancy the placenta has not yet developed sufficiently, so the amount of fetal DNA in maternal blood is extremely small. It becomes detectable by week 6 of pregnancy, but obtaining an accurate DNA test result requires the sufficient amount of DNA available from week 7 onward.

Q3. What happens if blood is drawn before 7 weeks of pregnancy?

A. There is a high risk that fetal DNA cannot be sufficiently extracted, requiring a retest. It would also fall outside the scope of seeDNA's free retesting and money-back guarantee, causing a financial disadvantage as well. To obtain an accurate result on the first attempt, please be sure to have blood drawn only after entering week 7 of pregnancy.

Q4. Is there individual variation in the amount of fetal DNA in maternal blood?

A. Yes, there is significant individual variation in the amount of fetal DNA. Factors such as maternal BMI (body mass index), placental condition, and the accuracy of the gestational week affect the amount, so even people at the same gestational week can have different amounts of fetal DNA. For this reason, whether a sufficient amount of fetal DNA is obtained cannot be known until the test is actually performed.

Q5. Is it okay to have blood drawn based on a gestational week I calculated myself?

A. Self-calculated gestational age based on menstrual cycle, last menstrual period, or ovulation date may not be accurate. Please always have an ultrasound scan at an obstetrics and gynecology clinic, have the gestational week confirmed by a doctor, and then decide the timing of the blood draw. Accurately determining the gestational week is an important point for improving the success rate of the test.

Q6. Is prenatal DNA testing possible in the case of an ectopic pregnancy or miscarriage?

A. In the case of an ectopic pregnancy, or if the fetus had already passed away at the time of the blood draw, fetal DNA cannot be detected in the maternal blood, so prenatal DNA testing cannot be performed. It is important to first confirm at an obstetrics and gynecology clinic that the pregnancy is a normal intrauterine pregnancy and that the fetal heartbeat has been confirmed, before having blood drawn.

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