GUIDE

Pregnancy Tests and Scans: The Complete Guide

Every routine test across the forty weeks, what each one is looking for, and how to read the report you are handed.

Reviewed by Dr. Grishma Ranjangaonkar, MBBS, DGO, ICOG Fellowship in Gynecological Endoscopy · Eva WomanCare Clinic, Vashi

Most antenatal anxiety comes from a report nobody explained. This guide goes through every routine test in order, sets out the difference between a test that diagnoses and one that only estimates a probability, and translates the abbreviations on a scan report into plain language.

Every routine test, plotted across the forty weeksA typical Indian antenatal schedule. Extra tests are added if anything needs watching, and the exact weeks vary between doctors.

The one distinction that explains everything

Antenatal tests come in two kinds, and almost all the confusion and most of the distress comes from mixing them up.

A diagnostic test tells you whether something is present. Your blood group, your haemoglobin, whether you have hepatitis B. The answer is yes or no and it is reliable.

A screening test does not. It gives you a probability, and it is designed to sort a large population into a lower-risk group and a higher-risk group so that further testing goes where it is most useful. The NT scan, the quadruple marker and NIPT are all screening tests. A high-risk result on any of them does not mean your baby has the condition. A low-risk result does not mean it is impossible.

Why this matters so much in practice A screening test reported as 1 in 150 sounds alarming. It means that out of 150 women with that exact result, one will have an affected baby and 149 will not. The number is doing its job by prompting a conversation about a diagnostic test, not by delivering a verdict. Nobody should be told a result like this without that explanation.

First trimester: weeks 1 to 12

Test	When	What it is looking for
Dating ultrasound	Weeks 6 to 10	Confirms the pregnancy is inside the uterus, checks the heartbeat, counts how many, and sets the due date to within about five days
Blood group and Rh factor	First visit	If you are Rh negative and the baby is Rh positive, you need anti-D injections to prevent problems in this and later pregnancies
Complete blood count	First visit	Mainly anaemia, which is very common in Indian pregnancies, plus platelet count and signs of infection
Thyroid function, TSH	First visit	Untreated hypothyroidism affects fetal brain development and is common and easily treated
HIV, hepatitis B and C, syphilis	First visit	All can be managed to protect the baby, but only if they are known about
Blood sugar	First visit	Picks up diabetes that predates the pregnancy, which needs tighter control than gestational diabetes
Urine routine and culture	First visit	Infection and protein. Urinary infections in pregnancy can reach the kidneys quickly
NT scan with the double marker	Weeks 11 to 13+6	Screening for chromosomal conditions. Measures fluid at the back of the neck and combines it with two blood markers and your age

The NT scan has a hard deadline The measurement is only valid between 11 weeks and 13 weeks 6 days. Miss the window and it cannot be done later; the alternative becomes the quadruple marker in the second trimester, which is a less accurate screen. If your dates are uncertain, this is a reason to get the dating scan done promptly.

Second trimester: weeks 13 to 28

Test	When	What it is looking for
Quadruple marker	Weeks 15 to 20	A blood screen for chromosomal conditions and neural tube defects, used when the NT scan window was missed
Anomaly scan	Weeks 18 to 22	A systematic examination of the brain, face, spine, heart, abdomen, kidneys and limbs, plus the placenta, cord and fluid
Glucose tolerance test	Weeks 24 to 28	Gestational diabetes, which usually causes no symptoms at all. South Asian women are at higher risk at lower body weights
Repeat haemoglobin	Around week 28	Anaemia commonly worsens as blood volume peaks
Anti-D injection	Around week 28	Given to Rh negative women whether or not there has been any bleeding
Blood pressure and urine	Every visit	Early signs of pre-eclampsia, which often produces no symptoms until it is advanced

Third trimester: weeks 29 to 40

Test	When	What it is looking for
Growth scan	Weeks 32 to 36	Estimated fetal weight, amniotic fluid volume, placental position and the position the baby is lying in
Doppler study	With the growth scan if indicated	Blood flow through the umbilical cord and fetal vessels, which shows how well the placenta is working
Group B streptococcus swab	Weeks 35 to 37	A common and harmless bacterium in you, which matters only because antibiotics during labour protect the baby
Non-stress test	From about 32 weeks if indicated	Records the fetal heart rate against movements for around twenty minutes, used for reduced movements or any growth concern
Repeat haemoglobin and blood pressure	Ongoing	Anaemia and pre-eclampsia, both of which can develop late

How to read a scan report

Scan reports are written for doctors and handed to patients, which is how a routine report ends up causing a week of worry. Here is what the abbreviations mean.

On the report	What it means
CRL	Crown-rump length. Used in the first trimester and the most accurate way to date a pregnancy
BPD	Biparietal diameter, the width of the head between the two parietal bones
HC	Head circumference
AC	Abdominal circumference. The measurement most sensitive to growth problems, because the liver shrinks first when nutrition is restricted
FL	Femur length, the thigh bone
EFW	Estimated fetal weight, calculated from the measurements above. It carries an error of roughly 10 to 15 per cent either way, which is why a scan cannot tell you the birth weight
AFI or DVP	Amniotic fluid index or deepest vertical pocket, two ways of measuring how much fluid there is
Presentation	How the baby is lying. Cephalic is head down, breech is bottom down, transverse is sideways
Placenta, anterior or posterior	Which wall of the uterus it is attached to. Anterior simply means you may feel movements later. What matters clinically is whether it is low-lying
Doppler, S/D or PI or RI	Numbers describing blood flow resistance in the cord. They are interpreted against the gestational age, not against zero

The table below gives the median measurements at each stage, from the WHO fetal growth charts, so you can see roughly where a report sits. The final column shows the tenth to ninetieth centile spread in weight, which is the honest picture: at 40 weeks, nine in ten healthy babies fall anywhere between about 3.1 and 4.1 kilograms.

Gestation	Median head circumference	Median femur length	Median weight	Weight, 10th to 90th centile
Week 16	123 mm	19 mm	144 g	124 to 166 g
Week 20	173 mm	32 mm	330 g	286 to 380 g
Week 24	222 mm	43 mm	665 g	576 to 765 g
Week 28	264 mm	52 mm	1,189 g	1,026 to 1,368 g
Week 32	296 mm	61 mm	1,901 g	1,635 to 2,187 g
Week 36	321 mm	69 mm	2,745 g	2,352 to 3,153 g
Week 40	342 mm	73 mm	3,617 g	3,084 to 4,135 g

What a high-risk screening result actually means

This is the single most distressing conversation in antenatal care, and most of the distress is avoidable. If your NT scan or quadruple marker comes back high risk, three things are true at once, and they need holding together.

It is a probability, not a finding. A result of 1 in 100 means 99 women out of 100 with that result have an unaffected baby.
It is calculated from your age, the scan measurement and blood markers combined. A woman of 38 can get a higher-risk number than a woman of 26 with identical measurements, purely because age is in the formula.
The next step is a choice, not an obligation. NIPT is a more accurate blood screen with no risk to the pregnancy. Amniocentesis or CVS give a definite answer but involve a small risk of miscarriage. Some women want certainty and some do not, and both are legitimate.

Ask for the number, not the word If you are told a screen came back high risk, ask what the actual ratio is and what the cut-off was. High risk usually means anything above a threshold such as 1 in 250, so a result of 1 in 240 and a result of 1 in 10 are both reported the same way and mean very different things.

What you can decline, and what it costs you

Every test here is offered rather than imposed, and you can decline any of them. That said, they are not equally weighted, and it is worth knowing which ones you would be giving up something real by refusing.

Test	What declining actually costs you
Anaemia and thyroid bloods	A great deal, for very little effort. Both are common in Indian pregnancies, both affect the baby, and both are simple to treat once known
Infection screening	The chance to prevent transmission to the baby. Hepatitis B in particular is largely preventable with treatment at birth
Dating scan	Accurate dates, which every later decision about growth, induction and prematurity depends on
Anomaly scan	The main opportunity to find structural problems in time to plan, including ones best delivered in a hospital with a paediatric surgical team
Glucose tolerance test	Detection of a condition that is usually symptomless and raises the risk of a very large baby and a difficult delivery
Chromosomal screening	Information, and nothing more. This is the one test where declining is a genuinely reasonable personal choice, and it depends entirely on what you would do with the result

Two things the law fixes for you

Sex determination is a criminal offence Under the PCPNDT Act, 1994, no scan centre, doctor or technician in India may determine or disclose the sex of a fetus, and no report may record it. The notice on the wall of the scan room is there for this reason. Anyone offering to tell you is committing an offence and should not be trusted with anything else either.

The second is more useful to you. Every scan centre must be registered and the report must carry that registration. You are entitled to copies of all your own reports, and you should keep them together, because whoever manages your delivery will want the whole set and not a summary.

Median head circumference, femur length and estimated fetal weight figures are taken directly from the WHO Fetal Growth Charts (Kiserud et al., PLOS Medicine 2017, CC BY 4.0). Test timings follow common Indian antenatal practice and vary between doctors. This guide is general information and does not replace your own doctor advice.

Guides and tools that go with this

First Trimester Guide

Weeks 1 to 12, week by week, including which tests belong in that window.

Second Trimester Guide

Weeks 13 to 28, with the anomaly scan and the glucose test in context.

Third Trimester Guide

Weeks 29 to 40, growth scans, movements and the signs of labour.

Due Date Calculator

Accurate dates are what every test window depends on. Work yours out from a scan or your last period.

Pregnancy Warning Signs

Sorted by urgency: what means go now, what means call today, and what can wait for your next visit.

Holding a report you do not understand?

Bring it in. A scan or blood report takes a few minutes to go through properly with someone who can see your whole record, and that is almost always faster than a week of searching for what a number means.

Questions about tests and reports

What is the difference between screening and diagnostic tests?

A diagnostic test tells you whether something is present, and the answer is reliable: your blood group, your haemoglobin, whether you carry hepatitis B. A screening test gives you a probability only, and is designed to sort a population into lower and higher risk groups. The NT scan, the quadruple marker and NIPT are all screening tests. A high-risk screening result is a prompt for a conversation, not a diagnosis.

My NT scan came back 1 in 150. What does that mean?

It means that out of 150 women with exactly that combination of measurements, blood markers and age, one will have an affected baby and 149 will not. It is above the usual cut-off, which is why it is reported as high risk, so the next step is offered: NIPT, which is a more accurate blood screen with no risk to the pregnancy, or a diagnostic test such as amniocentesis, which gives a definite answer but carries a small risk. Ask for the exact ratio and the cut-off used, because 1 in 240 and 1 in 10 are reported identically.

Can I skip the anomaly scan?

You can decline it, but it is the main opportunity to find structural problems while there is still time to plan. Some findings change where you should deliver, for example if a paediatric surgical team should be present. It is also worth knowing that it cannot detect everything and says nothing about most genetic conditions, so it is neither dispensable nor a guarantee.

The report says my baby is on the 30th centile. Is that a problem?

No. Centiles describe where a baby sits among healthy babies, and by definition half of all healthy babies are below the 50th. What matters is the trend across scans, not one number. A baby tracking steadily along the 30th centile is growing well. A baby that has dropped from the 60th to the 30th between two scans is a different picture, and that is what the second scan is for.

Why does the estimated fetal weight keep changing?

Because it is an estimate calculated from head, abdomen and femur measurements, and it carries an error of roughly 10 to 15 per cent either way. On a 3 kg baby that is around 400 grams in either direction. It is useful for spotting a trend across scans and for flagging babies who are very large or very small. It cannot predict the birth weight, and nobody should be planning a delivery around a single number.

Is the glucose drink safe, and can I eat before it?

It is safe, and it is unpleasant rather than difficult. Follow the fasting instructions you are given exactly, because eating beforehand invalidates the result and means repeating the whole thing. If you vomit the drink, tell the staff straight away rather than waiting, because the test will need rescheduling.

Will anyone tell me the sex of my baby?

No. Determining or disclosing fetal sex is a criminal offence in India under the PCPNDT Act, 1994. It will not appear on any report, no registered centre will disclose it, and anybody offering to is breaking the law.