I know that size at birth isn't necessarily a predictor of adult size, but has some link to size of the parents at birth. What about height at one year old?

My boy/ girl twins just had their one year check-up and vaccinations (poor things got two shots in each leg, as we wanted to catch up with the US vaccination schedule- they were born in Ireland). They are both tall, and I am not sure where it comes from! The boy is 31.8 inches (95 percentile) and the girl 29.9 inches (70 percentile); the boy is underweight at 21 pounds (doctor is not worried as he is healthy and active), the girl is 21.4 pounds, about average. They were average at birth at 22 and 21 inches and both their father and I are of average height at 5.8 foot for him and 5.6 foot for me. So... can we expect future basketball players?

How tall are the grandparents? Isn't that one of Mendel's findings, that we should look to the grandparents (of plants or humans and other animals) to explain these apparent aberrations?

Silja, there are lots of fun calculators online:

Child's Estimated Adult Height Calculator

How Tall Will Your Child Be?

I also found this answer from the Mayo Clinic:

Child growth: Can I predict my child's adult height?

Grandparents are average height too- his mom is on the small side of average, my mom on the tall side, but not hugely so.

An old wives tale says that at age 2, if you measure your child and multiply by 2, that will be the height they will be as adults.

That didn't work with either of my girls

Don't start on the old wive's tales, the links are bad enough. Could be a lot of paternity suits

Examples: a)daughter 5'9" son 5'10" [mother 5'1", father 5'11" ]
b)son 6' daughter 5' 8" [mother 5'4" father 5'5"] Those are both from my own family [b ) is particularly close to home ]

These eccentric results are traceable to a previous generation.The tall gene was lurking! In a) the daughter, now 57, is tall for her generation at 5'9".Though she has a tiny mother, her grandmother (her mother's mother)was extraordinarily tall for her generation [born 1882] at c. 5'10".Yet that woman's brother was only 5'3".Her own children, apart from the cited one, were son at 5'8", daughter at 5'7", daughter at 5'9".That oddity in the male line may explain the very short father in b) who is the grandson of that woman.He had little chance of being tall, since his own mother, at 5'1",used to joke that she was the tallest in her surviving family ( brother, sister and mother) if she were wearing heels, and may have come from a long line of Irish dwarfs (or jockeys, as they known over there )