The 12 week scan

What to expect in the 12 week scan?

In the UK, all pregnant women are offered two routine ultrasound scans. The first scan is performed between 8 and 14 weeks of pregnancy, typically around 12 weeks. This scan is offered alongside the combined screening test for chromosomal abnormalities. One of its main purposes is to estimate the gestational age of the baby, commonly referred to as a dating scan. This scan provides an estimated due date around which maternity care will be planned—though this does not mean the baby must be born by that exact date.

Another purpose of this early scan is to check for the number of babies in the womb, detect any major abnormalities, and confirm that the embryo is developing in the correct location. Ultrasound scans work by using sound waves to create an image of the baby. They are painless, and the official stance is that they have no known side effects. (For more on risks of scans check here).

You will be asked to drink plenty of fluids before the scan, as a full bladder helps lift the uterus out of the pelvis, improving image quality. The dating scan is usually performed with a probe pressed against the belly, but if the uterus is still too low, a transvaginal probe may be used. The procedure typically takes about 20 minutes and is conducted in a dimly lit room. Private clinics also offer dating scans for under £100.

Issues with dating scans

Dating scans estimate the gestational age of a baby by measuring the distance from the crown (top of the baby’s head) to the rump (bottom). This measurement is then matched to a standardized formula to determine gestational age. However, this method has limitations.

For instance, if a baby’s crown-to-rump length (CRL) is 55mm and corresponds to 12 weeks of gestation, how do we know whether the baby is truly 12 weeks old, or instead a large 11-week-old or a small 13-week-old? Another challenge is the lack of a universally accepted formula for dating pregnancies. Different hospitals use different formulas, and guidelines can vary depending on the provider. Furthermore, the accuracy of dating scans decreases as pregnancy progresses—so a due date may differ depending on who performs the scan, where it is conducted, when it is done, and even the baby’s position at the time of measurement.

This variability can be seen even in IVF pregnancies, where conception and embryo transfer dates are precisely known. Despite this, dating scans often assign a completely different due date. Additionally, healthcare professionals typically rely on the ultrasound-determined due date and may disregard other indicators or the mother’s own knowledge of her cycle.
For more details, you can review the evidence in this study: Link to study.

Lastly, a very important factor to consider is that the due date by scan is not accurate and can affect care in labour. If the date is moved back when you are certain about your conception/menstruation date, it can mean you are being offered an IOL for postdates earlier than you need, and if it is moved forward you may be treated as preterm if you go into labour early but not actually be preterm. This limit can affect your choice of place of birth, entail more discussions with care providers and be labelled as high risk. However, having a due date and an IOL date does not mean that you have to follow it.

What about using the last menstrual period?

Dating scans can be more accurate than menstrual dates, but only if performed in the first trimester. Menstrual dating may not be that accurate because it assumes ovulation and fertilization occur exactly 14 days after the first day of the last period. However, sperm can survive in the reproductive tract for up to five days before fertilizing the egg, making the exact timing of conception variable. Additionally, about 30% of women have irregular cycles, which can be influenced by conditions such as PCOS, recent miscarriage, or breastfeeding.

The use of menstrual dating originated in the 18th century when Dutch doctor Hermann Boerhaave devised a formula that later became known as Naegele’s Rule. He added 7 days and 9 months to the first day of the last menstrual period to estimate a due date. This method was widely adopted, particularly in legal paternity cases, but it is not highly accurate. More recent studies tracking hormone levels in urine suggest that the natural length of pregnancy often exceeds Naegele’s predicted due date. For a deeper look at the evidence surrounding due dates, I recommend reading the book In Your Own Time by Sarah Wickham.

All in all, trying to come up with a date that defines normal pregnancy length, current gestational age and a point around which to base interventions (postdates inductions, extra support for preterm births, etc.) is problematic and not rooted in high quality evidence. But it is helpful to have a rough date if you are hiring my services so I can be on call for the birth of your child in the right period!

Down Syndrome Screening

During the 12 week scan, women have the option to undergo screening for chromosomal conditions, including Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13). This screening does not provide a definitive diagnosis but assesses the likelihood of the baby having one of these conditions. It combines an ultrasound measurement of nuchal translucency (fluid at the back of the baby’s neck) with a blood test that examines specific hormone levels.

Some women choose to decline this screening for personal reasons, including a preference to avoid unnecessary anxiety or because they feel the results would not influence their pregnancy decisions. If the screening indicates a higher chance of a chromosomal condition, further diagnostic tests—such as chorionic villus sampling (CVS) or amniocentesis—may be offered. These tests provide conclusive results but carry a small risk of miscarriage. If these come back as positive you will be offered a termination of pregnancy. Some women do not contemplate this option so they decline this screening all together.

Women who opt out of the screening will still receive their routine dating scan and structural checks. Most healthcare providers respect and support individual choices for this aspect of the scan, ensuring that each woman receives care that aligns with her personal values and circumstances.

Benefits of the 12 week scan & what if you decline?

Many parents opt to go ahead with this first scan as it is the first external confirmation of pregnancy after the initial pregnancy home test. It is a landmark for many as, after a 12 weeks scan news are spread with family members and colleagues, since the risk of miscarriage drops significantly. It can be extremely reassuring to hear that baby is well and get a little glimpse into your bundle of joy. It can be beneficial for your mental health too if you have suffered a previous pregnancy lost. Some women, however, get a sense of safety and security from their spiritual believes, these been Christian, Muslin, New age, etc. and therefore decide to decline the 12 week scan.

Seeing your baby for the first time can be a bonding experience, a realisation that you truly are pregnant. So far pregnancy may feel like an illness as you have not felt baby move yet. Seeing your baby can help you bond with him/her, feel connected to your baby. You also get a picture of the baby usually at a small charge (£5). There are other ways to bond with baby even this early such as talking to your baby.

As mentioned earlier, determining the exact date of conception can be difficult, especially for women with irregular cycles or those who are unsure of their last menstrual period. While some women have a strong intuition about when conception occurred, this is not always the case.

Having an estimated due date can be beneficial, as it ensures that maternity care—such as blood tests, screening procedures, gestational diabetes testing (GTT), and vaccinations—is provided at the recommended stages of pregnancy. Additionally, certain interventions, like induction of labor, are often based on gestational age. However, it’s important to note that inductions are sometimes recommended not due to an immediate medical need but because of statistical risk factors, which may not apply to every individual pregnancy.

Accurate dating can also be critical in cases of extremely preterm labor. If a baby is born at 23–24 weeks, which is considered the threshold of viability, healthcare teams make decisions about neonatal care based on gestational age. Being in the appropriate hospital with the right level of neonatal support can make a significant difference in such situations.

Twin pregnancies remain relatively rare, with the likelihood of carrying twins estimated at approximately 1 in 250. However, due to the increased risks associated with multiple pregnancies, NICE guidelines recommend additional care, including regular consultant appointments and fortnightly scans for higher-risk cases starting from 14 weeks. Women expecting twins are also advised to increase their folic acid intake to support fetal development.

Complications in twin pregnancies can include twin-to-twin transfusion syndrome (TTTS), restricted growth, birth defects, and preterm birth. For the mother, risks include gestational hypertension, gestational diabetes, and anemia. While some complications cannot be avoided, many can be prevented or managed with early intervention. The risks are notably higher for twins that share a placenta, as they have a greater chance of developing serious complications.

In rare cases, twins remain undetected until birth, particularly in women who decline ultrasounds. Some late discoveries occur when unexpected pregnancy symptoms arise, as seen in this story. If routine scans are declined, an unusually large fundal height in the third trimester can be a clue. Additionally, an experienced midwife can often detect twins through abdominal palpation in the later stages of pregnancy.

During this scan the sonographer would look for baby’s heart rate to confirm that the pregnancy is viable. Sometimes they cannot find one. According to Tommy, the pregnancy and baby charity, 10 to 20% of known pregnancies end in miscarriage before 12 weeks. Sometimes a miscarriage is not detected until the 12 week scan, but the baby is found to have stopped growing a few weeks before. This is a missed miscarriage as there is usually no bleeding or cramping beforehand. The management for a miscarriage can either be expectant (to wait), medical or surgical. All of 3 carry the same risk of infection of 1-3:100. This means that if you do not have a scan and then miscarry you are not putting your health at risk more than someone that has the scan. You are just finding out later.

Around 1 on 80 (1.25%) are ectopic. This means that the fertilised egg has implanted somewhere other than uterus (for instance the fallopian tubes). Most of ectopic pregnancies happen before the 12 week scan, commonly around 6 weeks, and are usually discovered in AnE after an episode of sharp abdominal pain and heavy bleeding. It can come with pain on the tip of your shoulder and upset stomach (diarrhoea). Occasionally it can be picked up in a 12 week scan but most likely it would have happened before. It is a life threatening emergency. If you are over 35, your risk of having an ectopic pregnancy rises from under 2 out of 100 (under 35) to 4 out of 100 (over 35).

Traditionally women are screened for pre-eclampsia filling up a questionnaire. If they have several risk factors then they are recommended to start aspirin. However, some Trusts have now adopted the ASPRE screening. This screening combines the pressure on the baby’s umbilical cord through scan (dooplers) alongside the mothers blood pressure and details with a blood test for PAPP-A. An algorithm then issues a result as to whether the mother needs aspirin or not. St George Hospital reports a reduction of 25% in the cases of pre-eclamsia following this screening. This is still very new and you may not be offered it. Dooplers are also checked on the 20 week scan.

PAPP-A stands for Pregnancy Associated Plasma Protein A. This is protein produced by the placenta and when its levels are low you have a higher risk of developing pre-eclamsia and having a baby with low birth weight. Therefore more scans and a daily dose of aspirin is recommended to prevent pre-eclampsia. This protein is detected on the blood test done to check for chromosomal abnormalities alongside the 12 week scan.

Occasionally fibroids or ovarian cysts are found on the 12 week scan for the first time. They do not usually cause trouble but they will be measured again in the 20 week scan as fibroids can become problematic if they are near the entrance of the uterus or are of a significant size that could cause a heavier bleed after the birth of the baby.

Finally, some unexpected but major developmental issue with the fetus can be picked up in these early scans that are occasionally not compatible with life. For a more detail anatomy scan the 20 week scan is a better option.

What are the alternatives?

Alternatives to dating scans: go by your last menstrual period or focus on a due period (from 37 weeks to 42 weeks) rather than a due date. You can also decide on a cut off date by which or before which you would accept an intervention (posdates induction/preterm birth).

Alternatives ways to find a twin pregnancy: it can be detected on the third trimester by scan or possibly palpation if you abdomen is measuring larger than the average. Morning sickness may be worse.

Alternatives to bonding: building a connection with baby through feeling the movements, talking to baby, singing to baby.

Alternatives to viability and safety: develop your intuition to be able to tell when things are well or when something is wrong. If you develop any signs of infection (high temperature, smelly discharge) it may be worth considering whether you are having a miscarriage.

Alternatives ways to find an ectopic pregnancies: seek urgent medical attention if you are experiencing abdominal pain, vaginal bleeding, referred pain to your shoulder.

What does your intuition say?

What is your philosophy around the role of preventative medicine/screening in health?

How would you approach a smear test and other screening procedures?

How do you feel about the extra surveillance in pregnancy?

Do you feel pregnancy is a normal event that should go well or is it a time bomb waiting to go off?

Do you feel you need the external reassurance to know that everything is going well?

What if you do nothing?

Do nothing in this case means not having the scans/extra surveillance.

The due date might not be that relevant for you if you will not consider induction for postdates no matter what and are set on having a homebirth regardless of gestation.

With time, all of the complications that this scan picks up will be revealed. Such as a miscarriage, ectopic pregnacy, twin pregnancies, date of birth and even fetal abnormalities. Some later in pregnancy, some later at birth. Waiting may carry risk as discussed in the benefits section.

Midwifery Support

If you decline this scan, we will continue focusing on holistic, personalized care to ensure you feel confident and well-supported throughout your pregnancy. Whatever your decision, I’m here to support you, ensuring that you feel confident and well-informed throughout your pregnancy journey. If you choose to birth at home or decline interventions, not having scans will not affect the care you receive. As an independent midwife, I am happy to support you during a homebirth without scans or any other aspect of your pregnancy.

If you’re interested in my services, please check out the link below.

If you would like a midwife that can support you if you want to decline any scan in pregnancy, or if you want to have a chat about your options, why not book a discovery call with me?