Pregnancy blood tests: what, when, and why

During pregnancy, a woman undergoes more laboratory testing than at any other healthy period of her life — and the vast majority of those results come back normal. This entire screening system exists not to cause anxiety but to catch the rare situations where early intervention genuinely makes a difference: iron deficiency, thyroid dysfunction, gestational diabetes, an undetected infection. Identifying these conditions early produces measurably better outcomes for mother and baby.

One essential piece of background to absorb before you read your first trimester results: reference ranges for most blood tests change substantially during pregnancy. Plasma volume expands by 40–50%, the kidneys increase their filtration rate, and hormonal shifts ripple through virtually every measured parameter. Applying the standard non-pregnant reference table to a pregnant woman can lead to wrong conclusions in both directions — either missing a genuine problem or flagging a perfectly normal adaptation as abnormal.

This article provides an approximate testing roadmap by trimester so you know what to expect. Your obstetrician will adapt the schedule to your individual situation — pre-existing conditions, risk factors, and the results of your earlier appointments.

First trimester (weeks 4–13)

Most first-trimester laboratory work clusters around the booking visit — the first comprehensive antenatal appointment, typically between 8 and 12 weeks. The aim is to capture baseline health data before pregnancy has significantly altered your physiology.

Test	Purpose
Full blood count (FBC)	Baseline haemoglobin and haematocrit; white cell count reference point
Blood group + Rhesus + atypical antibodies	Critical for Rh-negative women — anti-D prophylaxis planning
HIV, syphilis, hepatitis B (HBsAg), hepatitis C	Universal screening per WHO 2016 recommendations
Rubella IgG	Assess immunity; vaccination is not possible during pregnancy
Urinalysis	Proteinuria baseline; asymptomatic bacteriuria screen
TSH	Screen for hypo- and hyperthyroidism; affects fetal neurological development
Ferritin	Assess iron stores before clinical anaemia develops
Fasting glucose or HbA1c (if risk factors present)	Early gestational or pre-existing diabetes screen

Full blood count (FBC) establishes a baseline. The key marker at this stage is haemoglobin: in the first trimester, before haemodilution has taken full effect, this is the best window to identify pre-existing anaemia and address it proactively.

Blood group and Rhesus status is essential if you have never had it documented. If you are Rh-negative and your baby’s father is Rh-positive, your obstetrician will arrange prophylactic anti-D immunoglobulin to prevent rhesus sensitisation.

Infectious disease screening — HIV, syphilis, hepatitis B, and hepatitis C — is offered to all pregnant women regardless of perceived risk, in line with WHO 2016 antenatal care recommendations. Rubella IgG assesses whether you have protective immunity; if not, vaccination must wait until after delivery.

TSH (thyroid-stimulating hormone) is an important early screen because both hypothyroidism and hyperthyroidism can affect fetal development. Read more about interpreting TSH results here. A critical nuance for pregnancy: in the first trimester, TSH naturally falls — sometimes to around 0.1 mIU/L — because human chorionic gonadotropin (hCG) cross-stimulates the thyroid. This means the first trimester has its own reference interval that differs substantially from the standard non-pregnant range. The exact trimester-specific limits vary by laboratory and measurement method; your clinician will use pregnancy-specific ranges, not the generic table. Untreated hypothyroidism during pregnancy is associated with impaired fetal neurodevelopment, so any abnormal TSH result warrants a specialist review.

Ferritin is increasingly measured at the booking visit — before clinical anaemia develops — to identify depleted iron stores while there is still time to replenish them. In pregnancy, a ferritin below 30 µg/L is commonly used as the threshold for iron deficiency, even when haemoglobin is still within the normal range. A low ferritin result at booking is a signal to begin iron supplementation, typically in discussion with your midwife or obstetrician.

Fasting glucose or HbA1c at booking is not offered to every woman — it targets those with risk factors: BMI ≥30, gestational diabetes in a previous pregnancy, a first-degree relative with type 2 diabetes, or polycystic ovary syndrome. The purpose is to exclude undiagnosed pre-existing diabetes before it can be labelled as gestational diabetes later in the pregnancy.

Second trimester (weeks 14–27)

Between 24 and 28 weeks comes the most important screening checkpoint of the entire pregnancy — the gestational diabetes test. By this stage, physiological insulin resistance has risen enough in many women that a hidden impairment of glucose metabolism becomes detectable.

Test	Timing	Purpose
75 g OGTT	24–28 weeks	Gestational diabetes screening
Full blood count (FBC)	24–28 weeks	Assess physiological dilutional anaemia
Aneuploidy screen (quadruple test / NIPT)	14–22 weeks	Chromosomal abnormality risk assessment
Repeat atypical antibody screen (Rh-negative)	28 weeks	Monitor for sensitisation

The 75 g oral glucose tolerance test (OGTT) is performed in the morning after an overnight fast of at least 8 hours. You drink 75 g of glucose dissolved in water and have blood drawn three times: before the drink, one hour after, and two hours after. The diagnostic thresholds and what they mean are covered in full in the gestational diabetes section below.

Full blood count at the end of the second trimester often shows a lower haemoglobin than at booking — this is expected physiology as plasma volume has expanded more than red cell mass. But not every fall is physiological: per WHO criteria, a haemoglobin below 105 g/L (10.5 g/dL) in the second trimester meets the definition of anaemia and warrants investigation and usually treatment.

Aneuploidy screening. The quadruple test (AFP, hCG, unconjugated estriol, inhibin A) is the biochemical component of second-trimester screening. It does not diagnose chromosomal conditions but estimates statistical risk. In many healthcare settings the quadruple test is now replaced or supplemented by non-invasive prenatal testing (NIPT), which analyses cell-free fetal DNA from the mother’s blood. The choice between approaches is made with your obstetrician.

Third trimester (weeks 28–40)

The third trimester is preparation for birth. The laboratory agenda shifts toward ensuring you are in the best possible shape for labour: checking blood status once more ahead of potential blood loss, screening for late infections, and preparing for operative delivery if needed.

Test	Timing	Purpose
Full blood count	≈28 weeks	Pre-labour anaemia check
Repeat HIV, syphilis	28–36 weeks	Per local protocol in higher-prevalence settings
Group B Streptococcus (GBS) swab	35–37 weeks	Prevent neonatal sepsis
Clotting screen + repeat blood group	If caesarean planned	Haemostasis assessment
Urinalysis (every visit)	Throughout third trimester	Pre-eclampsia surveillance (proteinuria)

Full blood count at around 28 weeks: anaemia in the third trimester is defined as haemoglobin below 110 g/L (11.0 g/dL) by WHO criteria. Identifying and correcting anaemia before labour gives you adequate time to respond to oral iron or, where necessary, intravenous iron therapy.

Group B Streptococcus (GBS) swab is not a blood test, but it is an integral part of late-pregnancy care in many protocols. GBS is a bacterium commonly found in the gut and vagina that causes no harm to the mother but can cause serious neonatal sepsis if passed to the baby during vaginal birth. Colonisation is not treated in advance; instead, intravenous antibiotics are given during labour.

Pre-eclampsia surveillance takes place at every third-trimester visit through blood pressure measurement and urine dipstick testing for protein. If proteinuria is detected, your obstetrician will arrange a 24-hour urine collection or a spot protein-to-creatinine ratio, along with a full blood count, liver enzymes, and creatinine. Pre-eclampsia is a serious condition; early detection is essential for both mother and baby.

Why pregnancy reference ranges differ

If a result in your antenatal report falls outside the standard printed reference range, do not assume something is wrong before asking whether the laboratory uses pregnancy-specific reference intervals — many do not print them automatically.

Plasma expansion dilutes the blood. Plasma volume increases by 40–50% while red cell mass rises by only 20–25%. The result is a predictable fall in haemoglobin, haematocrit, and other concentration-dependent values. This is physiological haemodilution — not genuine deficiency.

The kidneys hyperfiltrate. Glomerular filtration rate rises by 40–60% during pregnancy. As a consequence, serum creatinine falls 25–40% below non-pregnant levels. A creatinine that looks “normal” in an ordinary reference table — say, 70 µmol/L — may already indicate relative renal impairment in a pregnant woman. Pregnancy requires its own creatinine reference intervals.

TSH falls in the first trimester. Structurally similar to TSH, hCG cross-stimulates the thyroid gland. As hCG peaks in early pregnancy, TSH is physiologically suppressed. What would be flagged as subclinical hyperthyroidism outside pregnancy can be an entirely normal finding in weeks 8–12.

Coagulation shifts towards clotting. Fibrinogen, factor VII, factor VIII, and factor X all rise during pregnancy while protein S falls. This physiological hypercoagulability limits haemorrhage at delivery — but it also explains the increased thrombosis risk that obstetricians monitor.

Cholesterol rises 50–60%. This is a hormonally driven adaptation, not a cardiovascular risk signal in an otherwise healthy pregnant woman. Lipid profiles are not routinely used to screen for cardiovascular risk during pregnancy.

Practical bottom line. Never compare your pregnancy results against a standard non-pregnant reference table found online. Ask your laboratory or obstetrician for trimester-specific ranges — or at minimum, ensure whoever interprets your result knows the gestational age at the time of testing.

Gestational diabetes screening

Gestational diabetes mellitus (GDM) affects approximately 5–10% of pregnancies and usually produces no symptoms at all. Left unmanaged, it significantly raises the risk of: macrosomia (a large baby, increasing the likelihood of shoulder dystocia during delivery), pre-eclampsia, preterm birth, neonatal hypoglycaemia after delivery, and — looking further ahead — type 2 diabetes in the mother within the following decade.

When to screen. All pregnant women without pre-existing diabetes are offered the OGTT between 24 and 28 weeks. Women with risk factors (obesity, previous GDM, first-degree relative with type 2 diabetes, polycystic ovary syndrome) may be offered earlier screening — sometimes at the first booking visit.

One-step approach (75 g OGTT). This is the method recommended by the ADA and WHO and the most widely used internationally. It is performed in the morning after a minimum 8-hour fast. You should not smoke, eat, or exercise during the test.

Time point	Threshold (mmol/L)	Threshold (mg/dL)
Fasting	≥ 5.1	≥ 92
1 hour after glucose	≥ 10.0	≥ 180
2 hours after glucose	≥ 8.5	≥ 153

A single value meeting or exceeding the threshold at any time point is sufficient to diagnose gestational diabetes.

If fasting glucose is ≥7.0 mmol/L (≥126 mg/dL), or HbA1c is ≥6.5% at early screening, this points to pre-existing (overt) diabetes rather than gestational diabetes — the management approach is different and requires urgent specialist involvement.

Two-step approach. Some clinical programmes use a two-step protocol: a non-fasting 50 g glucose challenge test first, followed by a 100 g OGTT if the initial result is positive. This approach is more common in North America. Most current international guidance (ADA, WHO, IADPSG) favours the single-step 75 g OGTT because it identifies more women with GDM, including mild cases.

After delivery. GDM usually resolves after birth — but not always. Women with GDM are advised to have a repeat 75 g OGTT at 6–12 weeks postpartum to identify persistent diabetes. Beyond that, annual fasting glucose monitoring is recommended, as the lifetime risk of progressing to type 2 diabetes is substantially elevated compared with women who had uncomplicated pregnancies.

What to do when a result is out of range

An isolated, mildly abnormal result during pregnancy is a very common finding. In most cases it does not require urgent action and will be discussed at your next scheduled antenatal appointment.

Contact your obstetrician the same day if you notice:

Severe headache combined with visible swelling (face, hands) and protein detected in your urine — these three signs together are a warning for pre-eclampsia.
Haemoglobin below 80 g/L (8.0 g/dL) — severe anaemia requiring prompt assessment.
Markedly elevated liver enzymes (ALT, AST).
Symptomatic hypoglycaemia — cold sweat, trembling, confusion — with a blood glucose below 3.5 mmol/L.

In all other situations: print or screenshot the results, note the date and gestational week, and bring them to your next appointment. Try to avoid interpreting isolated abnormalities using general-population reference tables or internet searches — your clinician has the clinical context that an online source cannot provide.

How HealthLab helps during pregnancy

Over nine months of pregnancy, you accumulate dozens of lab results from different appointments and different laboratories. HealthLab does not replace antenatal care and does not interpret clinical findings — but it does help you keep every trimester’s results in one place and see trends across the full arc of your pregnancy.

HealthLab automatically recognises biomarkers from PDF lab reports issued by any laboratory — haemoglobin, ferritin, glucose, TSH, creatinine — and builds a timeline chart without any manual data entry. When you attend your next appointment, you can show your midwife or obstetrician the full trend at a glance, not just the most recent printout.

Download HealthLab on the App Store

Frequently Asked Questions

Can I decline any of the recommended tests during pregnancy?

Yes — you have the right to decline any test. But each recommended screen is included for a specific reason, and some (HIV, syphilis, hepatitis B) protect your baby directly, not just you. Before declining any test, have a conversation with your midwife or obstetrician about what that specific test detects and what the implications of not having it are. It is always a shared, informed decision — not a default refusal form.

My creatinine is 45 µmol/L — is that normal in pregnancy?

In most cases, yes. During pregnancy the kidneys increase their filtration rate significantly, and creatinine falls as a result — sometimes to 40–50 µmol/L or even lower. What would look “below normal” in a standard non-pregnant reference table is an expected physiological adaptation. Conversely, if your creatinine during pregnancy sits in the usual non-pregnant “normal” range — say, above 70–80 µmol/L — that may actually be relatively elevated for a pregnant woman and worth discussing with your clinician.

What does a 'positive' glucose test mean in pregnancy?

A positive glucose tolerance test means that one or more of your OGTT values met or exceeded the diagnostic threshold. This is not a diagnosis of type 1 or type 2 diabetes — it is gestational diabetes, a condition specific to pregnancy. Your obstetrician or a specialist team will discuss dietary changes, blood glucose self-monitoring, and — if needed — medication. The great majority of women with GDM deliver healthy babies after appropriate management. The important step is to follow the recommended plan and attend your follow-up appointments rather than waiting to see whether the result “sorts itself out.”

Do I need to fast before antenatal blood tests?

It depends on the test. The OGTT for gestational diabetes requires a strict overnight fast of at least 8 hours — no food, coffee, or anything other than water. Most other antenatal blood tests (full blood count, TSH, ferritin, urinalysis) do not require fasting, although they are typically scheduled for the morning. If you have a combined panel, check with the laboratory or your midwife which specific tests are included and whether any fasting preparation is needed — a quick call before your appointment can save you a second trip.

This article is for general information only and does not replace clinical advice. Any clinical decision during pregnancy should be made in partnership with your obstetrician or midwife.