Every pregnancy scan explained: what each one looks for

The standard schedule

Dating/viability scan (6-9 weeks): confirms the pregnancy is in the uterus, detects the heartbeat, counts the number of babies, and dates the pregnancy — early dating is the most accurate there is, and it sets the due date used for everything afterwards.

NT scan with double marker (11-14 weeks): measures the fluid at the back of the baby's neck and combines it with a blood test to estimate the risk of chromosomal conditions. It also checks early structure and confirms dating and chorionicity in twins.

Anomaly scan, also called TIFFA or level II (18-22 weeks): the most detailed scan of the pregnancy, examining every organ system head to toe, plus the placenta's position, amniotic fluid volume and cervical length. It takes 30-45 minutes for a reason.

Growth scan (usually 30-34 weeks, sometimes repeated): estimates the baby's weight, checks fluid and placental position and grade, and often includes Doppler studies of blood flow in the cord and the baby's vessels.

Reading the report

Common measurements: CRL (crown-rump length, used for early dating); BPD, HC, AC, FL (head, abdomen and thigh measurements used to estimate weight and growth); EFW (estimated fetal weight — accurate to within a few hundred grams, not exactly).

AFI or SDP: amniotic fluid measures. Low fluid (oligohydramnios) or high fluid (polyhydramnios) both prompt further assessment.

Placenta: its position (anterior, posterior, fundal — none of which matters much) and whether it is low-lying or covering the cervix (which does). A low-lying placenta at 20 weeks usually migrates upward as the uterus grows and is rechecked around 32 weeks.

Doppler results describe blood flow resistance in the cord and the baby's brain — abnormal flow is one of the earliest signs that a baby is not getting enough, and it is why growth scans include it.

The single most useful rule for reading a scan report: do not google the phrases overnight. Sonographers describe everything they see, most of it turns out to be normal variation, and interpretation is the obstetrician's job.

The law, and unnecessary scans

Under the PCPNDT Act, determining or disclosing the sex of a fetus is a criminal offence in India — for the doctor, the clinic and the person who asks. Every scan centre displays this, sonographers will not answer the question, and asking puts them in an impossible position.

The law exists because of India's skewed sex ratio and the practice of sex-selective abortion. It is enforced, and the penalties include imprisonment and loss of medical registration.

On the other side: routine ultrasound in pregnancy is safe, and there is no evidence of harm from the standard schedule. What is not useful is repeated scans without indication, including keepsake 3D or 4D scans marketed to families — they add cost and anxiety and answer no clinical question.

Extra scans are justified for: bleeding, pain, reduced movements, twins, growth concerns, a low-lying placenta, high blood pressure or diabetes, previous complications, and post-date monitoring. Your obstetrician will say why.

When to see a doctor

Any scan finding described as needing follow-up should be discussed with your obstetrician promptly — take the report to them rather than interpreting it yourself. If a scan raises a serious concern, referral to a fetal medicine specialist is the appropriate next step.

Go immediately, regardless of a normal recent scan: bleeding, severe abdominal pain, waters breaking or leaking, reduced fetal movements, severe headache with visual changes, or contractions before 37 weeks. A normal scan last week does not answer today's symptom.

This article is general information, not medical advice, and is no substitute for personal medical advice. For any decision about your or your child's health, please consult your doctor.

Frequently asked questions

Q: The scan says 'low-lying placenta'. Should I be worried? — At 18-22 weeks, usually not — most low-lying placentas move up as the uterus grows, and it is rechecked at around 32 weeks. Your doctor will advise on any restrictions in the meantime.

Q: My baby is measuring small. What happens now? — Usually a repeat scan in two to three weeks with Doppler, because the trend matters far more than one measurement. Reasons include dating, parental build, and genuine growth restriction — which is exactly what the monitoring is designed to catch.

Q: Are 3D and 4D scans worth it? — Medically, no — they add nothing to the standard assessment. As a keepsake, they are a personal choice. They cannot be used to determine sex either, and any centre offering that is committing an offence.

Q: How many scans are too many? — There is no evidence of harm from clinically indicated scans, and no benefit from unnecessary ones. If you are being asked to come every two weeks with no explanation, it is reasonable to ask what specifically is being monitored.