A–Z Glossary

Critical period

A critical period is a developmental window when forming fetal organs or body systems may be especially sensitive to harmful influences.

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Risk levels and critical weeks by trimester

Risk levels and critical weeks by trimester

Understanding gestational age and “critical weeks” Pregnancy is usually dated by gestational age, counted from the first day of the last menstrual period. This means gestational age is about two weeks ahead of embryonic or fetal age counted from conception. When clinicians say “8 weeks pregnant,” the embryo is typically about 6 weeks from conception, […]