Antenatal care is not just about listening to the heartbeat and measuring your bump. Each visit — and each scan or blood test ordered — has a specific clinical purpose tied to what's developmentally happening with your baby at that stage. Here is a trimester-by-trimester breakdown of what your obstetrician is checking and why.
The first ultrasound is typically a transvaginal scan performed at 6–8 weeks. Its primary purpose is to confirm an intrauterine pregnancy (ruling out ectopic), establish viability via fetal cardiac activity, and determine the gestational age through crown-rump length (CRL) measurement. CRL-based dating is the most accurate method for establishing your estimated due date (EDD) — more reliable than last menstrual period dating, particularly in women with irregular cycles.
A comprehensive blood panel is drawn at the first visit. Standard tests include:
NIPT analyses cell-free fetal DNA (cfDNA) circulating in maternal blood to screen for chromosomal conditions. It is available from 9–10 weeks of gestation. NIPT screens primarily for:
It is critically important to understand that NIPT is a screening test, not a diagnostic test. A high-risk result does not confirm the diagnosis — it indicates that a diagnostic test is needed. A low-risk result is highly reassuring but does not guarantee a normal karyotype. False positives occur, and no management decision should be made on NIPT alone without confirmatory testing.
The NT scan is performed between 11 and 13 weeks and 6 days. It measures the fluid accumulation at the back of the fetal neck (nuchal translucency) via ultrasound. An increased NT measurement is associated with chromosomal abnormalities and structural cardiac defects.
When combined with two maternal serum markers — PAPP-A (pregnancy-associated plasma protein A) and free β-hCG — and maternal age, the result is a risk calculation known as the First Trimester Combined Screen. This provides a personalised risk estimate for trisomy 21 and 18.
💡 Screening vs Diagnostic Tests: NIPT and the NT combined screen are screening tests — they estimate risk. CVS (chorionic villus sampling) at 11–14 weeks and amniocentesis at 15–20 weeks are diagnostic tests — they analyse fetal chromosomes directly and provide a definitive answer. Diagnostic tests carry a small procedural miscarriage risk (~0.5–1%). The decision to proceed to diagnostic testing is made jointly between the couple and their obstetrician based on screening results and personal values.
The detailed anatomy scan at 18–22 weeks is one of the most important assessments in pregnancy. It is a systematic survey of fetal structure, evaluating:
No scan can guarantee a structurally normal baby — some conditions are not detectable on ultrasound. However, the anomaly scan identifies a significant proportion of major structural abnormalities, enabling counselling, specialist referral, and birth planning when needed.
In women with risk factors for preterm birth — such as a prior preterm delivery, cervical surgery, or uterine anomaly — a transvaginal cervical length measurement may be offered. A cervical length of <25mm before 24 weeks is associated with significantly increased preterm birth risk and may prompt consideration of progesterone supplementation or a cervical stitch (cerclage).
The OGTT screens for gestational diabetes mellitus (GDM) — new-onset glucose intolerance in pregnancy. It involves a fasting blood glucose, followed by a 75g glucose drink, with blood taken at 1 hour and 2 hours.
GDM is diagnosed (WHO/IADPSG 2013 criteria) if any value meets or exceeds:
GDM affects approximately 15–20% of pregnancies in Singapore — higher than Western populations — owing to the metabolic risk profile in Asian women. Untreated GDM increases risk of macrosomia, shoulder dystocia, neonatal hypoglycaemia, and later maternal type 2 diabetes. Management is with dietary modification, and insulin or oral agents if glycaemic targets are not met.
A second full blood count at 28 weeks checks for iron-deficiency anaemia, which frequently develops in the second half of pregnancy due to increased fetal demand and haemodilution. Anaemia in late pregnancy is associated with preterm birth and low birth weight. Treatment with oral iron supplementation is commenced if indicated.
Routine growth scans are not performed at every third-trimester visit in low-risk pregnancies, but are indicated when there are clinical concerns. Common indications include:
GBS (Streptococcus agalactiae) is a bacterium carried in the vaginal and rectal flora of approximately 15–30% of women. It is generally harmless to the mother but can cause serious neonatal sepsis, pneumonia, and meningitis if transmitted during delivery.
A low vaginal and rectal swab is offered at 36–37 weeks. If positive, intrapartum IV benzylpenicillin (penicillin G) is administered at the onset of labour, effectively preventing neonatal GBS disease. Women with penicillin allergy require alternative antibiotic cover — this should be flagged to the obstetric team in advance.
A CTG records fetal heart rate patterns and uterine contractions via external sensors. It is not a routine test in uncomplicated low-risk pregnancies but is indicated for:
CTG interpretation requires trained clinical expertise — the trace is one piece of information evaluated alongside clinical context, and not a standalone test.
From 36 weeks, your obstetrician will confirm the fetal presentation (which part of the baby is lowermost in the uterus) by abdominal palpation and, if uncertain, ultrasound.
Third-trimester visits also open the conversation about birth preferences and, where clinically indicated, the timing and method of delivery. Common indications for induction of labour (IOL) include:
💡 Know your scans: Dating scan → anomaly scan → growth scan are distinct with different purposes. Not all pregnancies need growth scans. If your doctor hasn't offered a growth scan, that's often a sign things are going well — not an oversight.
References
ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus (2018, reaffirmed 2023)
NICE Guideline NG201: Antenatal Care (2021, updated 2023)
SMFM Consult Series: Medically Indicated Late-Preterm and Early-Term Deliveries (2019)
KKH SingHealth Antenatal Care Protocols and Clinical Pathways (2023)
ISUOG Practice Guidelines: Performance of First-Trimester Fetal Ultrasound Scan (2013, updated 2023)
WHO Recommendations on Antenatal Care for a Positive Pregnancy Experience (2016)
ACOG Practice Bulletin No. 797: Prevention of Group B Streptococcal Early-Onset Disease in Newborns (2020)