Obstetrics and Gynaecology chapters
Antenatal care: dating, EDD, booking investigations and screening
How pregnancy is dated and the EDD worked out, what is done at the booking visit, which blood and urine tests are sent, which infections are screened for, and how high-risk women are picked out early.
- Date the pregnancy from the LMP with Naegele's rule (+9 months +7 days), then confirm with a first-trimester crown-rump length scan, which wins if the two disagree.
- Booking tests: FBC, blood group and antibodies, MSU culture, HBsAg, syphilis, HIV, haemoglobinopathy screen and an offer of Down syndrome screening.
- Pick out risk early: low-dose aspirin for pre-eclampsia risk, OGTT at 24-28 weeks for GDM risk, anti-D for rhesus-negative women and SFH from 24 weeks for growth.
On this page
Overview
Antenatal care is the planned care a woman receives from conception to labour. Its aims are to:
- keep mother and baby healthy and pick out problems early (anaemia, pre-eclampsia, diabetes, poor fetal growth);
- screen for infections, blood disorders and fetal anomalies;
- sort women into low risk (midwife or community care) and high risk (consultant-led or shared care);
- give advice on diet, smoking, exercise, breastfeeding and the common minor symptoms of pregnancy.
The first and most important contact is the booking visit, ideally in the first trimester.
Dating the pregnancy
Naegele's rule
Pregnancy is counted from the first day of the last menstrual period (LMP), not from conception. The median length of pregnancy is 280 days (40 weeks).
EDD = first day of LMP + 9 calendar months + 7 days.
The rule assumes:
- a regular 28-day cycle;
- ovulation around day 14;
- a normal cycle (not just after stopping the combined pill or soon after another pregnancy or breastfeeding).
For a regular cycle over 28 days, add the extra days (cycle length minus 28) to the EDD.
LMP 10 January 2026, regular 28-day cycle
+ 9 months = 10 October 2026; + 7 days = 17 October 2026 (the same as LMP + 280 days)
Same LMP but a regular 35-day cycle: add 35 − 28 = 7 more days → 24 October 2026
LMP 10 January 2026; clinic visit 1 June 2026
Days elapsed = 142; 142 ÷ 7 = 20 weeks and 2 days → 20+2 weeks
Ultrasound dating
- Ultrasound dates are more accurate than a remembered LMP, because the day of ovulation varies more than once thought.
- Crown-rump length (CRL) is used from 10+0 to 13+6 weeks; the combined dating and screening scan is done when CRL is about 45-84 mm (around 11+3 to 13+6 weeks).
- Head circumference is used from 14 to 20 weeks.
- After 20 weeks fetuses grow at different rates, so a scan can no longer set the dates reliably.
- Once a scan has set the EDD, that date is used whatever the LMP says.
Accurate early dating prevents wrong labels of growth restriction later and unneeded induction for "post-dates". This is one reason late booking is a problem.
A scan after 20 weeks measures size, not age. Do not re-date a pregnancy from a third-trimester scan.
Gravidity and parity
- Gravida: total number of pregnancies, including the present one, however they ended.
- Para: births at 24 weeks or more (live or stillborn) plus live births at any gestation. Twins count as two for parity.
- Losses before 24 weeks are often written after a plus sign.
| Situation | Notation |
|---|---|
| First pregnancy, now 12 weeks | G1 P0 |
| Had twins last time, now pregnant again | G2 P2 |
| Six miscarriages, one live birth at 32 weeks, pregnant again | G8 P1+6 |
The booking visit
History
- Dates: LMP, cycle length, contraception before conception.
- Past obstetric history in date order: miscarriage, preterm birth, pre-eclampsia, abruption, growth restriction, stillbirth, big baby, congenital anomaly, and mode of each delivery (previous caesarean, difficult instrumental birth, PPH, severe perineal tear).
- Medical history: hypertension, diabetes, epilepsy, thyroid, heart and kidney disease, thromboembolism; current drugs.
- Family history: diabetes, hypertension, pre-eclampsia, inherited disease; consanguinity (relevant in Pakistan for recessive disorders such as thalassaemia).
- Social history: smoking, alcohol, drugs, support at home. See every woman alone at least once to ask safely about domestic abuse.
Examination
- Height, weight and BMI. BMI over 35 needs consultant review because the risks of pre-eclampsia, GDM, thromboembolism and anaesthetic difficulty rise.
- Blood pressure (also detects chronic hypertension early).
- Heart and breasts as indicated. Routine vaginal examination is not needed.
Booking investigations
| Test | Why it is done |
|---|---|
| Full blood count | Anaemia and low platelets. Anaemia = Hb below 110 g/L in the first trimester, below 105 g/L in the second and third, below 100 g/L after delivery |
| Blood group, Rh D and antibody screen | Find Rh D-negative women and women with atypical antibodies; repeat antibody check at 28 weeks |
| Haemoglobinopathy screen | Thalassaemia and sickle cell carriers; test the partner if she is a carrier |
| Midstream urine culture | Asymptomatic bacteriuria (linked to pyelonephritis and preterm birth) |
| Urine dipstick (every visit) | Protein (pre-eclampsia, renal disease), glucose (diabetes), nitrites (infection) |
| Hepatitis B surface antigen | Allows vaccine plus immunoglobulin for the baby at birth |
| Syphilis serology | Treatable with penicillin; prevents congenital syphilis and stillbirth |
| HIV | Antiretroviral treatment cuts vertical transmission to a very low level |
| Hepatitis C | Only in high-risk women (injecting drug use, HIV or HBV positive) in UK practice |
| Early glucose test or OGTT | Women with previous GDM (to find pre-existing diabetes) |
| Dating and first-trimester screening scan | Viability, dates, number of fetuses, chorionicity, nuchal translucency, gross anomalies |
Tumour markers such as CA-125 are not booking tests. Rubella immunity was a classic booking test; the UK stopped routine rubella screening in 2016, but many centres elsewhere still check it. A non-immune woman avoids contacts and gets MMR after delivery, never in pregnancy.
In Pakistan many units also check blood sugar, HCV antibody and rubella at booking because local prevalence differs; follow the current provincial or national guideline.
Infection screening in more detail
| Infection | Point to remember |
|---|---|
| Hepatitis B | Without prevention about 90% of babies of infected mothers become chronic carriers. Baby gets HBV vaccine plus one dose of HBIG within 12 hours, then further vaccine doses |
| Hepatitis C | Transmission about 5%, much higher with HIV co-infection; no routine UK screening |
| HIV | Start ART in pregnancy; caesarean if viral load is high near term; formula feeding where safe water is available |
| Syphilis | Treat with penicillin |
| Asymptomatic bacteriuria | Treat even without symptoms to prevent pyelonephritis |
| Group B streptococcus | Not routinely screened for in the UK |
Anti-D and the rhesus-negative woman
- Give anti-D immunoglobulin to an Rh D-negative woman (without anti-D antibodies) after any potentially sensitising event: amniocentesis, CVS, abdominal trauma, antepartum bleeding, external cephalic version, ectopic or molar pregnancy, surgical termination.
- Give it promptly, at the latest by 72 hours; some protection may still occur up to 10 days.
- Before 12 weeks it is needed only after ectopic, molar pregnancy, therapeutic termination, or bleeding that is heavy, repeated or painful.
- Doses in Ten Teachers: 250 IU before 20 weeks; at least 500 IU after 20 weeks, plus a Kleihauer test to measure the fetal bleed.
- Routine antenatal prophylaxis at 28 weeks (single dose) or at 28 and 34 weeks (two doses) covers silent bleeds.
- After delivery, give anti-D if cord blood shows the baby is Rh D positive.
"Rh-negative mum: 28, 34, birth, and any bleed within 72".
Ongoing visits and routine checks
The WHO model (2016) recommends at least eight antenatal contacts, with the first in the first 12 weeks and later contacts at 20, 26, 30, 34, 36, 38 and 40 weeks (WHO guidance on antenatal care, https://www.who.int/news/item/07-11-2016-new-guidelines-on-antenatal-care-for-a-positive-pregnancy-experience, checked 11 Nov 2026). Pakistani programmes follow this model; check the current national guideline for local schedules.
At each visit: blood pressure, urine for protein, symptoms, fetal movements, and from 24 weeks the fundal height.
| Gestation | Key task |
|---|---|
| 11-14 weeks | Dating scan, nuchal translucency, combined screening |
| 16-18 weeks | OGTT for women with previous GDM |
| 18-22 weeks (around 20) | Anomaly scan; uterine artery Doppler at 20-24 weeks in selected high-risk women |
| 24-28 weeks | 75 g OGTT for women with GDM risk factors; repeat FBC and antibodies at 28 weeks; anti-D |
| 36 weeks | Check lie and presentation; plan for birth |
| 40-41 weeks | Membrane sweeps (weekly from 40 weeks); offer induction after 41 completed weeks |
Symphysis-fundal height (SFH)
- Measure from the top of the symphysis pubis to the top of the fundus, every visit from 24 weeks, and plot on a (preferably customised) chart.
- From 20 to 36 weeks the SFH in cm roughly equals the weeks of gestation (about 1 cm per week).
- Larger than dates: wrong dates, multiple pregnancy, macrosomia, polyhydramnios, fibroids.
- Smaller than dates: wrong dates, fetal growth restriction, oligohydramnios.
- A low or falling SFH needs an ultrasound growth scan.
For a size-date discrepancy, the commonest cause is mistaken dates, which is why early ultrasound dating matters.
Identifying high-risk women
Pre-eclampsia: low-dose aspirin
Start aspirin 75 mg daily from early pregnancy until delivery if there is one high-risk factor or two or more moderate ones.
| High-risk factors | Moderate-risk factors |
|---|---|
| Hypertensive disease in a previous pregnancy | First pregnancy |
| Chronic kidney disease | Age over 40 |
| SLE or antiphospholipid syndrome | Pregnancy interval over 10 years |
| Type 1 or type 2 diabetes | BMI 35 or more at booking |
| Chronic hypertension | Family history of pre-eclampsia; multiple pregnancy |
Thyroid disease is not a pre-eclampsia risk factor for aspirin.
Gestational diabetes
- Risk factors: previous GDM, previous baby of 4.5 kg or more, BMI 30 or more, a first-degree relative with diabetes, and South Asian, black Caribbean or Middle Eastern origin.
- Offer a 2-hour 75 g OGTT at 24-28 weeks; with previous GDM test early (16-18 weeks) and repeat at 24-28 weeks if normal.
- Diagnosis: fasting 5.6 mmol/L or more, or 2-hour 7.8 mmol/L or more.
Because Pakistani women are of South Asian origin, every Pakistani woman has at least one GDM risk factor under these criteria.
Preterm birth and growth restriction
- Previous preterm birth or late miscarriage, multiple pregnancy and cervical surgery (cone biopsy): consider serial cervical length scans.
- Previous growth-restricted baby, smoking, chronic hypertension: serial growth scans.
Advice and supplements
- Folic acid 400 micrograms daily before conception and through the first trimester to prevent neural tube defects; 5 mg for higher-risk women (previous baby with an NTD, epilepsy on anticonvulsants, diabetes, inflammatory bowel disease).
- Vitamin D 10 micrograms daily in pregnancy and breastfeeding (UK advice).
- Iron: oral iron is first-line treatment for anaemia; routine supplements are common in Pakistan and in multiple pregnancy.
- Tetanus vaccination (TT or Td) is given to pregnant women under the national EPI programme in Pakistan; follow the current EPI schedule.
- Stop smoking (growth restriction, preterm birth, abruption, stillbirth) and avoid alcohol (fetal alcohol syndrome).
- Non-contact exercise is safe; avoid lying flat in late pregnancy (aortocaval compression).
- Encourage breastfeeding: WHO advises starting within an hour of birth and exclusive breastfeeding for 6 months.
Expect: calculating the EDD with Naegele's rule and adjusting for a long cycle; working out gestational age; CRL as the most accurate dating method and why scans after 20 weeks cannot re-date; gravida and para notation including twins; the list of booking tests and spotting the odd one out (CA-125); the Hb cut-offs for anaemia; asymptomatic bacteriuria screening; hepatitis B prophylaxis for the newborn; anti-D timing and routine 28-week prophylaxis; SFH equal to weeks from 20 to 36 and causes of large or small for dates (commonest: wrong dates); aspirin risk factors for pre-eclampsia; OGTT at 24-28 weeks; timing of the nuchal and anomaly scans; folic acid 400 micrograms versus 5 mg.
Quick revision
- EDD = LMP + 9 months + 7 days; add (cycle − 28) days for long cycles.
- Date by CRL at 10-14 weeks; HC from 14 to 20 weeks; never re-date after 20 weeks.
- Booking: FBC, group and antibodies, haemoglobinopathy, MSU, HBsAg, syphilis, HIV, dating scan.
- Anaemia: Hb under 110 (T1), 105 (T2-T3), 100 (postpartum) g/L.
- Rh-negative: anti-D at 28 (and 34) weeks, after any sensitising event within 72 hours, and after birth if baby is Rh positive.
- SFH from 24 weeks; cm ≈ weeks; large or small → scan.
- Aspirin 75 mg: one high or two moderate pre-eclampsia risk factors.
- OGTT 75 g at 24-28 weeks for GDM risk; fasting 5.6 or 2-hour 7.8 mmol/L.
- Anomaly scan around 18-22 weeks; WHO advises at least eight contacts.
Written by The House Job from standard references (Obstetrics by Ten Teachers).
FAQ
Why is ultrasound dating preferred over the LMP?
Ovulation does not always happen on day 14 and women often recall the LMP inaccurately, so a first-trimester crown-rump length gives a more reliable EDD.
Is a rubella test still part of booking?
The UK stopped routine rubella screening in 2016, but many Pakistani units still test; a non-immune woman is vaccinated after delivery, not during pregnancy.
Why screen for bacteria in the urine of a woman with no symptoms?
Untreated asymptomatic bacteriuria in pregnancy can progress to pyelonephritis and is linked with preterm birth, so it is treated when found.
What does a fundal height of 34 cm at 28 weeks suggest?
The uterus is larger than dates; the commonest reason is wrong dates, then multiple pregnancy, polyhydramnios, a large baby or fibroids, so arrange an ultrasound.