How Your Due Date is Calculated
The most widely used method - Naegele's Rule - calculates the Estimated Due Date (EDD) by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day menstrual cycle. If your cycle is regularly longer or shorter, the EDD is adjusted accordingly.
Naegele's Rule:
EDD = LMP + 280 days (or 9 months and 7 days)
Cycle adjustment:
EDD = LMP + 280 + (cycle length − 28) days
From conception date:
EDD = Conception date + 266 days (38 weeks)
[Conception typically = LMP + 14 days]
IVF - Day 5 blastocyst transfer:
EDD = Transfer date + 261 days
IVF - Day 3 embryo transfer:
EDD = Transfer date + 263 days
The Three Trimesters
First Trimester (Weeks 1-13): The most critical period for organ development. Morning sickness, fatigue, and breast tenderness are common. Key tests include nuchal translucency ultrasound (11-14 weeks) and first-trimester blood screening.
Second Trimester (Weeks 14-27): Often called the "golden trimester" - morning sickness usually subsides, energy returns, and the baby's movements become noticeable (quickening, around 18-20 weeks). The anatomy scan (18-20 weeks) checks for structural abnormalities.
Third Trimester (Weeks 28-40): Rapid baby growth, increased discomfort, and preparation for birth. Group B strep testing at 36 weeks, weekly doctor visits from 36 weeks, and labour preparation classes.
Key Pregnancy Milestones in India
Indian obstetric care follows the government's Antenatal Care (ANC) schedule under the Pradhan Mantri Surakshit Matritva Abhiyan (PMSMA):
Week 8-12: First ANC visit - registration, blood tests, urine tests, BP, weight, dating ultrasound
Week 11-14: NT scan (nuchal translucency) - chromosomal screening (Down syndrome risk)
Week 14-20: Second ANC visit - blood pressure monitoring, Hb, urine protein
Week 18-20: Anomaly scan (TIFFA) - detailed anatomy scan
Week 24-28: Glucose tolerance test - gestational diabetes screening (important in India: higher prevalence)
Week 28-32: Third ANC visit - growth scan, Rh antibody test
Week 34-36: Group B Strep (GBS) test, finalise birth plan
Week 36+: Weekly visits, baby's position assessment, cervical readiness evaluation
Normal Delivery Window
A "term" pregnancy is considered complete at 37-42 weeks. Deliveries before 37 weeks are preterm; after 42 weeks are post-term. The EDD marks the statistical midpoint (40 weeks) - only about 5% of babies arrive exactly on their due date. The normal delivery window is 39-41 weeks, with 37-42 weeks considered the full-term range.
Frequently Asked Questions
How accurate is the LMP due date calculation?
LMP-based calculation assumes ovulation occurs exactly 14 days after the first day of the last period. This is accurate for women with regular 28-day cycles, but less accurate for those with irregular cycles. First-trimester ultrasound (before 14 weeks) is significantly more accurate - it measures the baby's size (crown-rump length) and can adjust the due date if it differs from LMP by more than 5-7 days.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of the last menstrual period - so at the moment of conception, you are already considered 2 weeks pregnant. Fetal (embryonic) age counts from actual fertilisation and is about 2 weeks less than gestational age. All pregnancy weeks and milestones in medical care use gestational age. When your doctor says "you are 12 weeks pregnant," they mean 12 weeks from LMP, not from conception.
Is an IVF due date calculated differently?
Yes. For IVF pregnancies, the due date is calculated from the embryo transfer date, adjusted for the age of the embryo. A Day 5 blastocyst is 5 days old, so the EDD is calculated as transfer date + 261 days (266 − 5). A Day 3 embryo gives EDD = transfer date + 263 days. IVF due dates are generally very accurate since the exact fertilisation date is known. The gestational clock still starts from the equivalent LMP date for consistency with standard obstetric care.
What is the TIFFA scan and when is it done?
TIFFA (Targeted Imaging for Fetal Anomalies) is the detailed anatomy scan performed between 18 and 20 weeks in India. It examines the baby's brain, heart, spine, kidneys, limbs, and other structures for anatomical abnormalities. It also checks placental position, amniotic fluid level, and can often reveal the baby's sex. It is one of the most important scans during pregnancy.
What is the normal fetal heart rate during pregnancy?
A normal fetal heart rate during pregnancy ranges from 110-160 beats per minute (bpm). It is first detectable by vaginal ultrasound around 6 weeks of pregnancy. The rate is typically higher in early pregnancy (150-170 bpm) and stabilises between 120-160 bpm by the end of the first trimester. A rate below 100 bpm or irregular rhythm warrants prompt medical evaluation.
