Highest transmission of hepatitis B from mother to fetus occurs, if the mother is infected during:
1st trimester
IInd trimester
IIIrd trimester
At the time of implantation
The virus is transmitted by parenteral route, sexual contact, veical transmission and rarely breast milk. The risk of transmission to fetus ranges from 10% in 1st TM to as high as 90% in 3rd TM. It is specifically high (90%) from those mothers who are seropositive to hepatitis B surface Antigen(HBsAg) and e-antigen(HBeAg) Reference: Textbook of Dutta,9th edition, page 271
Medical, surgical and gynaecological illness complicating pregnancy