Pregnancy Overview

Pregnancy Overview

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Definition: Pregnancy is the condition between conception (fertilization of an egg by a sperm) and birth, during which the fertilized egg develops in the uterus. In humans, pregnancy lasts about 40 weeks (280–288 days), assuming conception occurs approximately 2 weeks after the Last Menstrual Period (LMP). This is the standard length of pregnancy based on a reliable LMP.
Gestational Age Classification

• First Trimester
• Second Trimester
• Third Trimester

First Trimester Roughly the first 13 weeks and 6 days of pregnancy. It is further divided into:

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Early First Trimester (<10 weeks gestation)

• I. Germinal Period: Fertilized egg travels and implants into the uterine wall. Implantation occurs 10–14 days after conception.
• II. Embryonic Period: From the 3rd through 9th week of pregnancy. Major organs and anatomical structures begin to form.

Late First Trimester (11.1 to 13.6 weeks) Also called the Period of the Fetus. All major organs continue rapid growth and begin to interconnect.

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Second Trimester (14 weeks to 27.6 weeks)

• Fetus assumes a distinct human appearance.
• Nails harden, and skin thickens.
• Eyelashes, eyebrows, and scalp hair appear during the 5th and 6th months.
• Fetal visual and auditory senses become functional.

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Third Trimester (28 to 40 weeks)

• Considered full-term after 37 weeks.
• Organ systems mature rapidly.
• Fetus prepares for birth.
• Reaches "age of viability"—can survive outside the uterus.
• Shows more organized gross motor activity and sleep/wake cycles.
• Toward the end of the 9th month, fetus is positioned head-down with limbs curled into the "fetal position."

Indications for a First-Trimester Sonogram

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• Confirmation of the presence of an intrauterine pregnancy
• Evaluation of a suspected ectopic pregnancy
• Defining the cause of vaginal bleeding
• Evaluation of pelvic pain
• Estimation of gestational (menstrual) age
• Diagnosis or evaluation of multiple gestations
• Confirmation of cardiac activity
• Imaging as an adjunct to chorionic villus sampling, embryo transfer
• Localization and removal of an intrauterine device
• Assessing for certain fetal anomalies, such as anencephaly, in high-risk patients
• Evaluation of maternal pelvic masses and/or uterine abnormalities
• Measuring the nuchal translucency (NT) when part of a screening program for fetal aneuploidy
• Evaluation of a suspected Hydatidiform mole

AIUM Practice Guidelines for a Second- and Third-Trimester Sonogram

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• Screening for fetal anomalies
• Evaluation of fetal anatomy
• Estimation of gestational (menstrual) age
• Evaluation of fetal growth
• Evaluation of vaginal bleeding
• Evaluation of abdominal or pelvic pain
• Evaluation of cervical insufficiency
• Determination of fetal presentation
• Evaluation of suspected multiple gestation
• Adjunct to amniocentesis or other procedure
• Evaluation of a significant discrepancy between uterine size and clinical dates
• Evaluation of a pelvic mass
• Evaluation of a suspected Hydatidiform mole
• Adjunct to cervical cerclage placement
• Suspected ectopic pregnancy
• Suspected fetal death
• Suspected uterine abnormalities
• Evaluation of fetal well-being
• Suspected amniotic fluid abnormalities
• Suspected placental abruption
• Adjunct to external cephalic version
• Evaluation of premature rupture of membranes and/or premature labor
• Evaluation of abnormal biochemical markers
• Follow-up evaluation of a fetal anomaly
• Follow-up evaluation of placental location for suspected placenta previa
• History of previous congenital anomaly
• Evaluation of the fetal condition in late registrants for prenatal care
• Assessment for findings that may increase the risk for aneuploidy