A 0.5 cm embryo corresponds to approximately 5–6 weeks of gestation.

        A crown-rump length (CRL) of 0.5 cm corresponds to approximately 7 weeks of gestation, or about 49 days since conception. Gestational age can be estimated from the CRL using the formula: CRL (in cm) + 6.5. The integer part of the result represents the gestational age in weeks, and multiplying that week number by 7 yields the approximate number of days since conception. Details are as follows:

        CRL (cm) + 6.5 — This formula is generally applicable only up to 12 weeks of gestation. However, some women may find discrepancies between the gestational age calculated using this formula and their actual number of days since the last menstrual period (LMP). Such discrepancies commonly occur in the following two scenarios:

        1. Irregular Menstrual Cycles

        Irregular menstrual cycles can lead to earlier or later ovulation, resulting in correspondingly earlier or later implantation of the fertilized egg. Consequently, the calculated gestational age based on CRL may differ from the duration since LMP. For women with irregular cycles, early-pregnancy ultrasound measurements—such as CRL or gestational sac size—provide a more accurate assessment of gestational age.

        2. Poor Embryonic Development

        In rare cases, suboptimal embryonic quality may cause delayed embryonic development, thereby impairing normal growth of the embryo (including CRL). As a result, the gestational age estimated using the above formula may not align with the duration since LMP.