Antenatal care — Australian shared care schedule, first and second trimester screening

1. Definition and clinical relevance

Antenatal shared care is a model in which a pregnant person's routine pregnancy visits are distributed between their general practitioner and an obstetrician or midwifery team, with hospital-based care reserved for higher-complexity encounters. In Australia, most metropolitan and regional centres operate formal shared care programs, making this the dominant model of antenatal care delivery that an intern will encounter on obstetric and general practice terms. Understanding the schedule, the rationale for each screening test, and the thresholds that trigger escalation is essential: missed screening windows cannot always be reopened, and the consequences of delayed diagnosis (chromosomal anomaly, pre-eclampsia, Rhesus sensitisation) are serious. The first and second trimesters contain the highest density of time-critical screening decisions.

2. Key values, thresholds, scoring systems

3. Approach: presentation and differential

History at booking (before 10 weeks)

A thorough booking history covers: last menstrual period and cycle regularity, obstetric history (gravidity, parity, previous pregnancy complications), medical history (diabetes, hypertension, autoimmune disease, epilepsy, cardiac disease), medications and teratogen exposure, family history of chromosomal conditions or inherited disease, smoking and substance use, mental health, domestic violence screening, and country of origin (relevant to haemoglobinopathy risk and infectious disease screening).

Red flags that shift the patient out of low-risk shared care include: prior pre-eclampsia or preterm birth, pre-existing diabetes or hypertension, multiple pregnancy, BMI above 40, chronic kidney disease, significant cardiac disease, or a previous baby with a chromosomal or structural anomaly.

First trimester (weeks 6 to 13)

The main clinical concerns are: confirming viability and dating (early ultrasound), chromosomal screening (CFTS), nausea and vomiting severity, thyroid status, and identification of high-risk features requiring aspirin prophylaxis. Differential diagnoses for first trimester bleeding include threatened miscarriage, ectopic pregnancy, and subchorionic haematoma.

Second trimester (weeks 14 to 27)

Key concerns shift to: structural anomaly detection (morphology scan at 18 to 20 weeks), cervical length in those with prior preterm birth, anaemia, and gestational diabetes screening. Fetal movement is first perceived at around 18 to 20 weeks in nulliparous women (slightly earlier in multiparous women). Differential for second trimester pain includes round ligament pain, urinary tract infection, appendicitis (with displaced anatomy), and preterm labour.

Differential for abnormal screening results

An elevated AFP on MSS raises the possibility of open neural tube defects, anterior abdominal wall defects (gastroschisis, omphalocele), or multiple pregnancy, in addition to Down syndrome risk calculation. A low PAPP-A on CFTS is associated with increased risk of fetal growth restriction and placental dysfunction beyond chromosomal risk alone.

4. Investigations

Booking bloods (first visit)

Full blood count: establishes baseline haemoglobin and platelet count; iron deficiency anaemia is common. Blood group and Rh(D) status: blood group is documented for transfusion planning; Rh(D)-negative women require prophylactic anti-D to prevent formation of antibodies that could cause haemolytic disease in a future Rh(D)-positive pregnancy. Antibody screen: detects pre-formed red cell antibodies. Rubella serology: confirms immunity. Syphilis serology (RPR or TPPA): mandatory in most Australian states. HIV serology. Hepatitis B surface antigen: identifies neonates requiring immunoprophylaxis at birth. Hepatitis C antibody. Varicella immunity if history uncertain. Urine microscopy and culture: asymptomatic bacteriuria occurs in around 5% of pregnancies and, if untreated, carries a significant risk of ascending pyelonephritis and preterm labour. Thyroid function: offered to symptomatic women or those with risk factors.

Chromosomal screening

Combined first trimester screening (CFTS) is the preferred approach when the patient presents before 14 weeks. It integrates the NT measurement (performed between 11 and 14 weeks by a credentialled sonographer), free beta-hCG, and PAPP-A into a computer algorithm that also incorporates maternal age to generate an individual risk figure for trisomy 21 and trisomy 18. A risk above the programme cut-off prompts an offer of diagnostic testing (chorionic villus sampling or amniocentesis). An increased NT with normal chromosomes warrants detailed follow-up ultrasound, as it is associated with cardiac malformations and other structural anomalies.

For women presenting after 14 weeks, second trimester MSS (quad screen) is offered. This measures AFP, unconjugated oestriol, hCG subunits, and inhibin A. The algorithm again incorporates maternal age and gestational age. Approximately 5% of women will receive a screen-positive result; the majority of these women will have unaffected pregnancies. If AFP is included in the panel, the test simultaneously screens for open neural tube defects.

Cell-free fetal DNA (cfDNA, NIPT) is increasingly available and offers higher sensitivity and specificity for trisomy 21 than either CFTS or MSS alone, but it remains a screening test and a positive result requires confirmatory diagnostic testing.

Morphology ultrasound (18 to 20 weeks)

This is the primary structural survey. It assesses fetal anatomy systematically, including a four-chamber cardiac view and outflow tract assessment, neural tube integrity, abdominal wall, renal anatomy, limb structure, placental position, and amniotic fluid volume. It also confirms gestational age if not established earlier.

Haemoglobinopathy screening

Offered to women from high-risk populations (South-East Asian, South Asian, Mediterranean, Middle Eastern, African, and Pacific Islander backgrounds) to detect thalassaemia and sickle cell trait. If the woman is a carrier, partner testing is arranged; if both partners carry a clinically significant variant, chorionic villus sampling or amniocentesis can confirm fetal status.

Gestational diabetes screening

A 75 g oral glucose tolerance test (OGTT) at 24 to 28 weeks is standard. Earlier testing (first trimester) is offered to women with prior GDM, polycystic ovary syndrome, BMI above 30, or a first-degree relative with type 2 diabetes.

5. Management

Routine shared care visit schedule (low-risk pregnancy)

Visits are typically scheduled at: booking (before 10 weeks), 12 to 13 weeks (post-CFTS results, aspirin initiation if indicated), 16 weeks (MSS results review if applicable, uterine size, wellbeing), 20 weeks (post-morphology scan review), 24 weeks, 28 weeks (anti-D for Rh-negative women, OGTT results, FBC recheck), 30 to 32 weeks, 34 weeks (second anti-D dose for Rh-negative women), 36 weeks, 38 weeks, 40 weeks, and 41 weeks (post-dates discussion, referral for induction).

Pre-eclampsia prophylaxis

Women with one or more high-risk features (prior pre-eclampsia, chronic hypertension, pre-existing diabetes, renal disease, autoimmune disease, multiple pregnancy) or two or more moderate-risk features are offered aspirin 150 mg orally each night, started at 12 weeks gestation and continued until 36 weeks. This dose reduces the risk of early-onset pre-eclampsia.

Rh(D) prophylaxis

Rh(D)-negative women without pre-formed anti-D antibodies receive routine antenatal anti-D immunoglobulin at 28 weeks and again at 34 weeks. Additional doses are given after any sensitising event (antepartum haemorrhage, amniocentesis, external cephalic version, abdominal trauma, or miscarriage). Free fetal DNA testing from maternal blood can now identify whether the fetus is Rh(D)-positive, allowing targeted prophylaxis rather than universal administration.

Nutritional supplementation

Folic acid is the most time-critical supplement: standard-risk women take 500 mcg daily from at least one month before conception through to 12 weeks. Women at higher risk of neural tube defects (personal or family history of NTD, antiepileptic drug use, pre-existing diabetes, or BMI above 35) take 5 mg daily over the same period. Iodine 150 mcg daily is recommended throughout pregnancy and breastfeeding. Iron supplementation is guided by FBC and ferritin results.

Smoking and substance use

Smoking cessation is a priority at every visit. Brief intervention, referral to Quitline (13 7848), and nicotine replacement therapy are all appropriate in pregnancy. Alcohol has no established safe level in pregnancy. Opioid dependence should prompt referral to a specialist antenatal drug and alcohol service; methadone or buprenorphine maintenance is safer than continued illicit use.

Escalation criteria

Refer promptly to obstetric care for: blood pressure at or above 140/90 mmHg, proteinuria with hypertension, fetal growth restriction on ultrasound, abnormal fetal heart rate pattern, reduced fetal movements with clinical concern, antepartum haemorrhage, symptoms of preterm labour (regular contractions before 34 weeks), or any screen-positive result requiring diagnostic follow-up. In rural and remote settings, early liaison with retrieval services and telehealth obstetric consultation is essential before the clinical situation deteriorates.

6. Australian-specific considerations

Aboriginal and Torres Strait Islander women experience higher rates of preterm birth, low birthweight, gestational diabetes, and perinatal mortality than non-Indigenous women. Culturally safe antenatal care means engaging with community-controlled health organisations, involving family and community support networks where the woman wishes, and avoiding assumptions about family structure or social circumstances. Yarning-style consultations, use of Aboriginal health workers or liaison officers, and flexible appointment structures improve engagement. Indigenous-specific antenatal programs exist in many jurisdictions and should be actively connected to rather than bypassed.

In rural and remote areas, women may need to travel significant distances for NT ultrasound or morphology scans. Planning these appointments early, coordinating with visiting specialist services, and arranging accommodation support (through programs such as the Patient Assisted Travel Scheme, PATS, in most states) reduces the risk of missed screening windows. Telehealth obstetric review is appropriate for many routine shared care visits and for results discussion.

Refugee and migrant women may arrive with no prior antenatal records, incomplete vaccination histories, and higher prevalence of haemoglobinopathies, infectious diseases (hepatitis B, HIV, syphilis, tuberculosis), and female genital mutilation. A trauma-informed approach and use of professional interpreters (not family members) are standard. Haemoglobinopathy screening is particularly important in women from South-East Asia, the Pacific, and sub-Saharan Africa.

Medicare rebates support shared care visits under standard item numbers. The 715 Aboriginal and Torres Strait Islander Health Assessment can be used in the antenatal period to structure a comprehensive review and generate a care plan.

Clinical pearls

• The NT measurement window closes at 13 weeks 6 days. If a patient presents late in the first trimester, book the ultrasound immediately rather than waiting for the next available routine appointment.

• A screen-positive result on CFTS or MSS is not a diagnosis. Around 95% of screen-positive women carry an unaffected fetus. Careful counselling before testing, and again at the time of results, prevents unnecessary distress and uninformed decisions about diagnostic procedures.

• Low PAPP-A on CFTS (below the 5th centile) is a marker of placental dysfunction independent of chromosomal risk. These women warrant additional growth surveillance in the third trimester even when the chromosomal risk calculation is reassuring.

• Rh(D)-negative women must receive anti-D after every sensitising event throughout pregnancy, not only at the routine 28- and 34-week doses. A single missed dose after a first-trimester procedure can cause sensitisation that affects all future pregnancies.

• Aspirin for pre-eclampsia prevention must be started by 16 weeks at the latest to be effective. Identify high-risk women at the booking visit and prescribe before the CFTS results appointment.

• Elevated maternal serum AFP on second trimester MSS should prompt detailed ultrasound review of the fetal spine and anterior abdominal wall, not just a repeat blood test. Open neural tube defects and abdominal wall defects are the principal structural causes.

Sources

• AIHW Aboriginal and Torres Strait Islander health reports
• RACGP Red Book, Guidelines for preventive activities in general practice
• Therapeutic Guidelines (eTG)