Question 1

How is the due date calculated?

Accepted Answer

The estimated due date (EDD) is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This formula has been standard in obstetrics since the 19th century and assumes a regular 28-day cycle with ovulation on day 14. An alternative phrasing of the same rule: subtract 3 months from the LMP and add 7 days. For an LMP of January 10: subtract 3 months = October 10, add 7 days = October 17 (estimated due date). The 280-day count includes approximately 2 weeks before conception, which is why gestational age always runs 2 weeks ahead of fetal age.

Question 2

How accurate is the due date?

Accepted Answer

Only approximately 4% of babies are born on their exact calculated due date. Normal full-term delivery spans weeks 37 to 42 of gestation — a 5-week biological window. Early first-trimester ultrasound (ideally between 8–12 weeks) is more accurate than LMP dating, with a margin of ±5–7 days versus ±14 days for LMP alone. When LMP-based dating and first-trimester ultrasound agree within 7 days, clinical practice uses the LMP date. When they differ by more than 7 days, most guidelines recommend accepting the ultrasound-derived EDD as more accurate, particularly for women with irregular cycles.

Question 3

What are the three trimesters and what happens in each?

Accepted Answer

First Trimester (weeks 1–13): The embryonic period — all major organs form by week 10. The heart begins beating around week 6. The highest risk of miscarriage occurs in this period (approximately 10–15% of recognized pregnancies). First prenatal visit typically occurs at weeks 8–10. First-trimester screening (nuchal translucency ultrasound + blood tests for chromosomal conditions) is offered at weeks 11–13. Second Trimester (weeks 14–27): Generally the most comfortable period. Nausea often resolves. The fetus begins moving (felt by mother around weeks 18–22). The anatomy scan at week 20 checks fetal development in detail. Third Trimester (weeks 28–40+): Rapid fetal growth; lungs mature (by week 36). Weekly prenatal visits begin around week 36. Full-term threshold is 37 weeks; the due date is 40 weeks.

Question 4

What does 'weeks pregnant' really mean?

Accepted Answer

Gestational age ('weeks pregnant') is counted from the first day of the last menstrual period — not from conception. Since ovulation and conception typically occur around day 14 of the cycle, you are technically counted as 2 weeks pregnant at the moment of conception. A positive pregnancy test (usually detectable 4–5 weeks from LMP, about 2 weeks after conception) typically means you are already 4–5 gestational weeks pregnant. This counterintuitive counting system is universal in obstetrics because the LMP date is reliably known, while the exact conception date usually is not.

Question 5

What if I don't know my last menstrual period date?

Accepted Answer

If you are unsure of your LMP, several alternatives exist. First, try to recall your approximate last period — even knowing the week is sufficient for an initial estimate within ±7 days. Second, use a conception date if you know it (select Conception Date mode). Third, use a known due date from a prior ultrasound or prenatal visit (select Known Due Date mode). Fourth, your healthcare provider can perform a first-trimester ultrasound to date the pregnancy directly from fetal crown-rump length — this is the most accurate method when LMP is unknown and is standard practice in the first prenatal visit when the date is uncertain.

Question 6

What is the difference between this pregnancy calculator and the due date calculator?

Accepted Answer

Both calculators use Naegele's rule and produce the same core due date, gestational age, and trimester results. The due date calculator also supports IVF-specific transfer date methods (day-3 and day-5 embryo transfer) which are more accurate for IVF pregnancies. This pregnancy calculator supports a 'Known Due Date' input mode — useful when you already have an EDD from an ultrasound and want to work backwards to find the LMP or track your milestones from a confirmed date. Use whichever calculator's input method matches the date information you have.

Question 7

When should I have my first prenatal appointment?

Accepted Answer

The first prenatal visit is ideally scheduled at 6–8 weeks gestation (about 2–4 weeks after a missed period, or 4–6 weeks after conception). At this visit, your provider confirms the pregnancy, establishes the EDD (via LMP and/or early ultrasound), screens for risk factors, discusses prenatal vitamins (folic acid is critical in the first 12 weeks for neural tube development), and sets the schedule for all subsequent screenings. Waiting until 12 weeks risks missing the optimal window for first-trimester chromosomal screening (nuchal translucency, done at 11–13+6 weeks) and delays establishing accurate dating.

Question 8

What is a normal amount of weight to gain during pregnancy?

Accepted Answer

The American College of Obstetricians and Gynecologists (ACOG) recommends gestational weight gain based on pre-pregnancy BMI. For normal BMI (18.5–24.9): 11.5–16 kg (25–35 lb) total. For overweight BMI (25–29.9): 7–11.5 kg (15–25 lb). For obese BMI (≥30): 5–9 kg (11–20 lb). For underweight BMI (<18.5): 12.5–18 kg (28–40 lb). Weight gain should be gradual — approximately 0.5–2 kg in the first trimester, then 0.3–0.5 kg per week in the second and third trimesters. Excessive gestational weight gain is associated with higher rates of cesarean delivery, gestational diabetes, and postpartum weight retention.

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