Whether you went to work or just went to the bathroom, your baby now understands that his favorite person in the world sometimes leaves. And he's not too thrilled about it.

According to Dr. Tovah Klein, head of the Barnard College Center for Toddler Development and author of How Toddlers Thrive, many babies at this stage begin to show signs of separation anxiety and stranger anxiety. She explains how baby is:

More aware of unfamiliar people and places and may cry when someone new holds him (even if he's usually a social butterfly).

Klein recommends giving your baby extra time to warm up to new situations.

If you're going to leave him with a family member, friend or caregiver, be sure to stick around for a bit to ease him into the adjustment process.

When he feels comfortable, he'll act more like himself and play.

Week-by-week activities

And speaking of play, child development psychologist Dr. Holly Ruhl shares fun week-by-week activity ideas for month nine.

Week 1

Baby may hone motor and social skills by rolling a ball back and forth. Show your cutie how the pros play at a real ball game! Your tot will love the songs, mascots, fireworks, and halftime events. Keep baby happy with snacks, sunscreen and a tour of the stadium. Even free little league games offer baby a chance to behold an intriguing interplay between other children!

Week 2

Locomotor skills are related to infants' spatial search skills. This week, practice stepping movements and exercise those tiny leg muscles. Support your barefoot babe under the arms during practice steps, bounce your standing sweetie on the lap, or provide assistance as baby cruises along sturdy furniture. Supervise use of a safe push toy to promote independent mobility.

Week 3

Infants as young as 9 months can perform numerical computations with numbers up to 10, even before they can verbalize these skills! Introduce addition and subtraction into your daily routine. Add finger foods to baby's tray one by one, count down from 10 as you trim those tiny fingernails, or sing “Ten Little Bubbles" as you blow bubbles for baby to pop.

Week 4

Baby can imitate your actions even after a 24-hour delay! Exercise baby's developing memory by modeling desirable behaviors for your babe to imitate. Use a spoon, brush your teeth, share toys or care for stuffed animals. Now might also be the time to sidestep behaviors that you don't want emulated, so wait until baby's nap for the chocolate splurge and Pinterest marathon!

A treat for baby: VTech Sit-to-Stand Learning Walker

Whether your tot is practicing standing or walking, or would rather just plop down and play, this baby walker has it all. The removable panel is covered with interactive activities including piano keys, colorful spinning rollers, shape sorters, songs, sound effects, and light up buttons to develop fine motor skills.