How is a due date calculated?

The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle and ovulation on day 14. For cycles other than 28 days, add or subtract the difference. Example: LMP of January 1 → due date is October 8 (280 days later). Ultrasound-based dating, which measures fetal size, is most accurate in the first trimester.

What are the three trimesters of pregnancy?

The first trimester spans weeks 1 through 13 (conception through end of week 13). The second trimester covers weeks 14 through 26. The third trimester runs from week 27 through birth. Each trimester brings major developmental milestones. The first trimester is when all major organ systems form. The second is rapid growth and when the fetus becomes capable of movement the mother can feel. The third is lung maturation and weight gain in preparation for birth.

What is gestational age vs fetal age?

Gestational age is counted from the first day of the last menstrual period (LMP), even though conception typically occurs about 2 weeks later. Fetal age (or embryonic age) counts from the actual date of conception. A fetus that is 10 weeks gestational age is approximately 8 weeks old from conception. Medical providers use gestational age as the standard because the LMP date is known while the exact conception date is usually not.

When do you find out the baby's sex?

Sex can be determined via cell-free fetal DNA blood test (non-invasive prenatal testing, NIPT) as early as 10 weeks of gestational age. An anatomy ultrasound at 18 to 20 weeks can visually confirm sex. Chorionic villus sampling (CVS) at 10 to 13 weeks and amniocentesis at 15 to 20 weeks can determine sex with certainty as part of genetic testing, but these are invasive procedures typically reserved for genetic screening.

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Due date · Gestational age · Trimesters · Milestones · Week-by-week schedule

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How Due Dates Are Calculated

The most widely used method is Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes ovulation occurs on day 14 of a 28-day cycle, so for cycles of different lengths the ovulation day and therefore conception date shifts, and the due date adjusts accordingly.

EDD = LMP + 280 days Naegele's rule, standard medical practice For non-28-day cycles: EDD = LMP + 280 + (cycle length - 28) days Example: 35-day cycle → EDD = LMP + 287 days From conception date: EDD = conception date + 266 days (38 weeks)

IVF Due Date Calculation

For IVF pregnancies, the embryo's age at transfer is known exactly. The equivalent LMP is calculated by subtracting the embryo's age in days plus 14 days from the transfer date, then adding 280 days to that LMP.

Day 5 transfer: EDD = transfer date + 261 days Day 3 transfer: EDD = transfer date + 263 days Day 6 transfer: EDD = transfer date + 260 days

Ultrasound Dating

First-trimester ultrasound (crown-rump length measurement at 7 to 13 weeks) is the most accurate dating method, typically within 5 to 7 days. Second-trimester anatomy scans are accurate to about 1 to 2 weeks. Ultrasound dating takes precedence over LMP-based dating when the two differ by more than 7 days in the first trimester or more than 14 days in the second trimester.

The Three Trimesters

Trimester	Weeks	Key Developments
First	1 to 13	All major organ systems form. Heart begins beating at week 6. Neural tube closes by week 6-7. Most miscarriage risk occurs here.
Second	14 to 26	Rapid growth. Movement felt (quickening) at 16-22 weeks. Sex determinable by ultrasound at 18-20 weeks. Viability threshold around 22-24 weeks.
Third	27 to 40+	Lung maturation, fat deposition, brain development. Baby settles head-down. Most births occur 37 to 41 weeks.

Term Definitions

Term	Gestational Age	Notes
Premature (preterm)	Before 37 weeks	Requires NICU support depending on how early
Early term	37 0/7 to 38 6/7	Slightly higher risk than full term
Full term	39 0/7 to 40 6/7	Optimal window for labour
Late term	41 0/7 to 41 6/7	Monitoring intensified
Post-term	42 weeks or later	Induction typically recommended

ℹ️

Fewer than 4% of births occur on the exact due date. About 60% occur within one week and nearly 90% within two weeks. The EDD is a statistical midpoint of a normal distribution of birth timing, not a deadline.

Week-by-Week Development Summary

Weeks	Fetal Development	Mother's Body
4 to 5	Embryo implants. Neural tube, heart, and gut begin forming. Size of a poppy seed.	Missed period. Home pregnancy test positive. Early nausea may start.
6 to 7	Heart begins beating. Brain and face structures forming. Arm and leg buds appear. ~1 cm long.	Morning sickness peaks for many. Fatigue, breast tenderness, frequent urination.
8 to 10	Fingers and toes distinct. Eyelids form. Organs in place. Now called fetus. ~3 cm.	Uterus size of large orange. First OB visit typically occurs here.
11 to 13	Genitals developing. Bone starts hardening. Fingernails form. ~7-8 cm, 14-25 g.	Nuchal translucency scan and first-trimester screening. Risk of miscarriage drops significantly.
14 to 16	Facial muscles move. Sex organs developed. Hearing begins. ~12 cm, 100 g.	Second trimester energy often returns. Baby bump visible in many women.
17 to 20	Vernix caseosa (protective coating) forms. Quickening: mother feels movement. ~25 cm, 300 g.	Anatomy scan at 18-20 weeks checks organs and determines sex. Braxton-Hicks may begin.
21 to 24	Viability threshold (~22-24 wks). Lungs produce surfactant. ~30 cm, 600 g.	Glucose tolerance test at 24-28 weeks screens for gestational diabetes.
25 to 28	Eyes open. Responds to sounds. ~35 cm, 900 g. Brain growing rapidly.	Third trimester begins at week 27. Rhogam injection if Rh-negative.
29 to 32	Fat deposits increasing. Bone marrow producing red blood cells. ~40 cm, 1.5-1.8 kg.	Prenatal visits every 2 weeks. Heartburn and back pain common.
33 to 36	Lungs nearly mature. Head-down position usually assumed. ~45-47 cm, 2-2.5 kg.	Group B Strep test at 36 weeks. Pelvic pressure as baby descends.
37 to 40	Full term. Lungs mature. ~50 cm, 3-3.5 kg average at birth. Ready for delivery.	Cervix softening and dilating. Nesting instinct common. Labour signs to watch.

Prenatal Care Schedule

Weeks	Visit / Test	Purpose
8 to 10	First prenatal visit	Confirm pregnancy, blood type, Rh factor, STI screen, due date ultrasound
10 to 13	NIPT / nuchal scan	Chromosomal screening (Down syndrome, trisomy 18, sex)
11 to 14	First trimester screen	Blood tests for hCG and PAPP-A, combined with nuchal translucency
15 to 20	Quad screen	Blood test for neural tube defects and chromosomal issues
18 to 20	Anatomy ultrasound	Major organ check, placenta location, fetal sex
24 to 28	Glucose tolerance test	Gestational diabetes screening
28	Rhogam injection	If Rh-negative blood type
36	Group B Strep swab	Tests for GBS bacteria requiring antibiotic treatment during labour
36 to 40	Weekly visits	Cervical checks, fetal position, blood pressure monitoring

Frequently Asked Questions

How accurate is the due date calculation?

LMP-based dating is accurate within ±1 to 2 weeks for most people but assumes regular 28-day cycles and ovulation on day 14. First-trimester ultrasound (crown-rump length) is the most accurate dating method, within 5 to 7 days. Fewer than 4% of births occur on the exact due date. Most occur within one week before or after. The due date is a statistical midpoint, not a predicted delivery day.

What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of the last menstrual period, which is approximately 2 weeks before conception in a 28-day cycle. Fetal age counts from actual fertilisation. At 10 weeks gestational age, the embryo is approximately 8 weeks old from conception. All medical providers use gestational age as the standard because the LMP is usually known while the exact date of ovulation and fertilisation is not.

What is the viability threshold?

Viability refers to the gestational age at which a premature baby has a reasonable chance of survival outside the womb with intensive medical support. The current threshold in most developed countries is approximately 22 to 24 weeks. At 22 weeks survival rates are 10 to 35% and outcomes vary significantly by facility. At 24 weeks survival rates are 50 to 70%. At 28 weeks they exceed 90%. These numbers improve continuously as neonatal intensive care advances.

When is the highest risk of miscarriage?

The risk of miscarriage is highest in the first trimester, particularly before week 10. Approximately 80% of all miscarriages occur in the first 12 weeks of pregnancy. After a heartbeat is detected on ultrasound at 6 to 8 weeks, the risk drops significantly. After week 12, miscarriage risk falls to below 2% in the general population. Second-trimester pregnancy loss (14 to 20 weeks) affects approximately 1 to 2% of pregnancies and is sometimes called a late miscarriage.