6 weeks. A bimanual examination is helpful is estimating the gestational age. At 7 weeks gestation the uterus is about the size of a large hen's egg.
At 10 weeks gestation the uterus if about the size of an orange and at 12 weeks gestation the uterus is about the size of a grapefruit. After 16 weeks and up to 36 weeks gestation, the fundal height is centimeters is roughly equal to the number of weeks gestation with a singleton pregnancy.

Fetal heart tones may be detected with Doppler devices at about 12 weeks gestation. Non-electronic fetoscopes may ausculate fetal heart tones initially at 18 to 20 weeks.

Quickening (fetal movement) The pregnant patient feels fetal movement at approximately 16 to 18 weeks gestation

Usual timing and frequency of prenatal visits include:
Preconception Up to 1 year before conception
First prenatal 6 to 8 weeks after missed menses
Monthly Up to 28 weeks
Bimonthly 28 to 36 weeks
Weekly 36 weeks to delivery