ANTEPARTUM | TRIMESTERS | Maternity

Pregnancy Duration

A pregnancy lasts about 40 weeks (280 days) counted from the first day of the last menstrual period (LMP) and is divided into three trimesters. Accurate dating guides every prenatal assessment, screening test, and decision about the timing of birth.

GESTATIONAL AGE VS. FETAL AGE

Gestational age is counted from the first day of the LMP and is used clinically. Fetal (conceptional) age is counted from conception and is about 2 weeks less than gestational age.

DATING METHODS: The LMP is the starting point, but a first-trimester ultrasound (crown-rump length) is the most accurate way to confirm or adjust the due date.

Pregnancy Trimesters

Trimester	Weeks	Development Highlights
First	Weeks 1–13	Organ formation (organogenesis) — the period of greatest risk from teratogens; neural tube closes early (about week 6 of gestation); cardiac activity visible on transvaginal ultrasound at about 6 weeks; FHR heard by Doppler at about 10–12 weeks
Second	Weeks 14–27	Quickening (16–20 weeks); lanugo and vernix caseosa appear; fetal sex usually visible on ultrasound at about 18–20 weeks; surfactant production begins late in this trimester
Third	Week 28 to birth	Lung maturation; fat deposition; rapid weight gain; fetal descent into the pelvis (lightening) late in pregnancy

Trimester week ranges vary slightly between textbooks.

THINK SIMPLE

First trimester = building the organs (avoid teratogens). Second = the mother feels movement. Third = the baby grows, gains fat, and matures the lungs.

Pregnancy Timeframes

Term	Definition
Preterm	Before 37 0/7 weeks
Early term	37 0/7 to 38 6/7 weeks
Full term	39 0/7 to 40 6/7 weeks
Late term	41 0/7 to 41 6/7 weeks
Post-term	42 0/7 weeks or more

Fundal Height Landmarks

Gestational Age	Fundal Position
12 weeks	Just above the symphysis pubis
16 weeks	About halfway between the symphysis pubis and the umbilicus
20 weeks	At the umbilicus
20–36 weeks	Fundal height in centimeters ≈ weeks of gestation (± 2 cm)
36 weeks	Near the xiphoid process; may drop slightly after lightening

NCLEX TRAP: A fundal height more than 2 cm above or below the expected value between 20 and 36 weeks needs follow-up. Larger than expected: incorrect dates, multiple gestation, polyhydramnios, or a large fetus. Smaller than expected: incorrect dates, fetal growth restriction, or oligohydramnios.

Naegele’s Rule — Estimated Date of Birth

FORMULA

First day of LMP + 7 days − 3 months + 1 year

Start with the first day of the LMP

Confirm the exact date at the first prenatal visit.

Add 7 days

Move forward 7 days on the calendar.

Subtract 3 months

Count back 3 months from that date.

Add 1 year (if needed)

Adjust the year so the date falls about 9 months after the LMP.

LIMITATIONS: Naegele’s rule assumes a regular 28-day cycle with ovulation on day 14. For longer or shorter cycles, the date is adjusted by the difference in days. Near the end of a month, different calculation methods can differ by a day or two. The estimated date is only an estimate — most births occur within 2 weeks of it.

Practice Calculator

First day of LMP

Cycle length (days)

Enter the first day of the LMP and select Calculate.

The calculator adds 280 days (adjusted for cycle length) to the LMP. Use it to check your manual answers.

NCLEX Self-Check — Trimesters & Naegele’s Rule

Select the best answer for each question. Press Submit Answer to view your result and the rationale for every option.

Question 1 of 10

Why Dating Matters

Accurate dating determines when screening tests are done, whether a birth is preterm or post-term, and whether fetal growth is appropriate. An incorrect due date can lead to unnecessary interventions or missed complications.

Clinical Judgment Connections

Clinical Judgment	Application
Recognize Cues	Obtain the exact LMP, cycle length, and fundal height.
Analyze Cues	Calculate gestational age and compare fundal height with dates.
Prioritize Hypotheses	Identify size-date discrepancies and possible preterm or post-term status.
Generate Solutions	Plan ultrasound confirmation and timing of screenings.
Take Action	Report discrepancies and teach the client about her due date.
Evaluate Outcomes	Track fundal height and fetal growth at each visit.

NCLEX KEY POINT

LMP + 7 days − 3 months + 1 year. Fundal height in cm ≈ weeks from 20 to 36. Quickening at 16–20 weeks. Post-term is 42 weeks or more.